a. a. a. Jay L. Devore Probability and Statistics

Bibliography Devore, Jay, and Kenneth Berk, Modern Mathematical Statistics with Applications, Thomson-BrooksCole, Belmont, CA, 2007. A bit more sophisticated exposition of probability topics than in the present book. Olkin, Ingram, Cyrus Derman, and Leon Gleser, Probability Models and Applications 2nd ed., Macmillan, New York, 1994. Contains a careful and comprehensive exposition of joint distributions, rules of expectation, and limit theorems. are connected in parallel across a battery with voltage X 4 . Then by Ohm’s law, the current is Let m 1 10 ohms, s 1 1.0 ohm, m 2 15 ohms, s 2 1.0 ohm, m 3 20 ohms, s 3 1.5 ohms, m 4 120 V, s 4 4.0 V. Calculate the approximate expected value and standard devia- tion of the current suggested by “Random Samplings,” CHEMTECH, 1984: 696–697. 94. A more accurate approximation to E[hX 1 , . . . , X n ] in Exercise 93 is h m 1 , c ,m n 1 1 2 s 1 2 a 2 h x 1 2 b 1 c1 1 2 s n 2 a 2 h x n 2 b Y 5 X 4 c 1 X 1 1 1 X 2 1 1 X 3 d Compute this for Y hX 1 , X 2 , X 3 , X 4 given in Exercise 93, and compare it to the leading term hm 1 , . . . , m n . 95. Let X and Y be independent standard normal random vari- ables, and define a new rv by U .6X .8Y. a. Determine CorrX, U. b. How would you alter U to obtain CorrX, U r for a specified value of r? 96. Let X 1 , X 2 , . . . , X n be random variables denoting n inde- pendent bids for an item that is for sale. Suppose each X i is uniformly distributed on the interval [100, 200]. If the seller sells to the highest bidder, how much can he expect to earn on the sale? [Hint: Let Y maxX 1 , X 2 , . . . , X n . First find F Y y by noting that Y y iff each X i is y. Then obtain the pdf and EY.] Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 239 6 Point Estimation INTRODUCTION Given a parameter of interest, such as a population mean or population pro- portion p, the objective of point estimation is to use a sample to compute a number that represents in some sense a good guess for the true value of the parameter. The resulting number is called a point estimate. In Section 6.1, we present some general concepts of point estimation. In Section 6.2, we describe and illustrate two important methods for obtaining point estimates: the method of moments and the method of maximum likelihood. m Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. DEFINITION Statistical inference is almost always directed toward drawing some type of conclu- sion about one or more parameters population characteristics. To do so requires that an investigator obtain sample data from each of the populations under study. Conclusions can then be based on the computed values of various sample quantities. For example, let m a parameter denote the true average breaking strength of wire connections used in bonding semiconductor wafers. A random sample of n ⫽ 10 connections might be made, and the breaking strength of each one determined, resulting in observed strengths x 1 , x 2 , . . . , x 10 . The sample mean breaking strength could then be used to draw a conclusion about the value of . Similarly, if s 2 is the variance of the breaking strength distribution population variance, another parame- ter, the value of the sample variance s 2 can be used to infer something about s 2 . When discussing general concepts and methods of inference, it is convenient to have a generic symbol for the parameter of interest. We will use the Greek letter for this purpose. The objective of point estimation is to select a single number, based on sample data, that represents a sensible value for . Suppose, for example, that the parameter of interest is m, the true average lifetime of batteries of a certain type. A random sample of n ⫽ 3 batteries might yield observed lifetimes hours x 1 ⫽ 5.0, x 2 ⫽ 6.4, x 3 ⫽ 5.9. The computed value of the sample mean lifetime is ⫽ 5.77, and it is reasonable to regard 5.77 as a very plausible value of —our “best guess” for the value of based on the available sample information. Suppose we want to estimate a parameter of a single population e.g., or s based on a random sample of size n. Recall from the previous chapter that before data is available, the sample observations must be considered random variables rv’s X 1 , X 2 , . . . , X n . It follows that any function of the X i ’s—that is, any statistic—such as the sample mean or sample standard deviation S is also a random variable. The same is true if available data consists of more than one sample. For example, we can represent tensile strengths of m type 1 specimens and n type 2 specimens by X 1 , . . . , X m and Y 1 , . . . , Y n , respectively. The difference between the two sample mean strengths is ⫺ , the natural statistic for making inferences about 1 ⫺ 2 , the difference between the population mean strengths. m m Y X X m m m x u u m x A point estimate of a parameter is a single number that can be regarded as a sensible value for . A point estimate is obtained by selecting a suitable sta- tistic and computing its value from the given sample data. The selected statis- tic is called the point estimator of . u u u In the battery example just given, the estimator used to obtain the point estimate of m was , and the point estimate of m was 5.77. If the three observed lifetimes had instead been x 1 ⫽ 5.6, x 2 ⫽ 4.5, and x 3 ⫽ 6.1, use of the estimator would have resulted in the estimate ⫽ 5.6 ⫹ 4.5 ⫹ 6.13 ⫽ 5.40. The symbol “theta hat” is customarily used to denote both the estimator of and the point estimate resulting from a given sample. Thus is read as “the point estimator of is the sample m m ˆ 5 X u uˆ x X X 6.1 Some General Concepts of Point Estimation Following earlier notation, we could use an uppercase theta for the estimator, but this is cumber- some to write. ⌰ ˆ Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Example 6.1 mean .” The statement “the point estimate of m is 5.77” can be written concisely as . Notice that in writing , there is no indication of how this point estimate was obtained what statistic was used. It is recommended that both the esti- mator and the resulting estimate be reported. An automobile manufacturer has developed a new type of bumper, which is sup- posed to absorb impacts with less damage than previous bumpers. The manufacturer has used this bumper in a sequence of 25 controlled crashes against a wall, each at 10 mph, using one of its compact car models. Let X ⫽ the number of crashes that result in no visible damage to the automobile. The parameter to be estimated is p ⫽ the proportion of all such crashes that result in no damage [alternatively, p ⫽ Pno damage in a single crash]. If X is observed to be x ⫽ 15, the most reasonable esti- mator and estimate are ■ If for each parameter of interest there were only one reasonable point estima- tor, there would not be much to point estimation. In most problems, though, there will be more than one reasonable estimator. Reconsider the accompanying 20 observations on dielectric breakdown voltage for pieces of epoxy resin first introduced in Example 4.30 Section 4.6. 24.46 25.61 26.25 26.42 26.66 27.15 27.31 27.54 27.74 27.94 27.98 28.04 28.28 28.49 28.50 28.87 29.11 29.13 29.50 30.88 The pattern in the normal probability plot given there is quite straight, so we now assume that the distribution of breakdown voltage is normal with mean value . Because normal distributions are symmetric, is also the median lifetime of the distribution. The given observations are then assumed to be the result of a random sample X 1 , X 2 , . . . , X 20 from this normal distribution. Consider the following esti- mators and resulting estimates for : a . Estimator ⫽ , estimate ⫽ ⫽ ⌺x i n ⫽ 555.8620 ⫽ 27.793 b . Estimator ⫽ , estimate ⫽ ⫽ 27.94 ⫹ 27.982 ⫽ 27.960 c . Estimator ⫽ [minX i ⫹ maxX i ]2 ⫽ the average of the two extreme lifetimes, estimate ⫽ [minx i ⫹ maxx i ]2 ⫽ 24.46 ⫹ 30.882 ⫽ 27.670 d . Estimator ⫽ tr10 , the 10 trimmed mean discard the smallest and largest 10 of the sample and then average, estimate ⫽ tr10 ⫽ 27.838 Each one of the estimators a–d uses a different measure of the center of the sample to estimate . Which of the estimates is closest to the true value? We cannot answer this without knowing the true value. A question that can be answered is, “Which esti- mator, when used on other samples of X i ’s, will tend to produce estimates closest to the true value?” We will shortly consider this type of question. ■ m 5 555.86 2 24.46 2 25.61 2 29.50 2 30.88 16 x X x | X | x X m m m estimator pˆ 5 X n estimate 5 x n 5 15 25 5 .60 uˆ 5 72.5 m ˆ 5 5.77 X Example 6.2 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. The article “Is a Normal Distribution the Most Appropriate Statistical Distribution for Volumetric Properties in Asphalt Mixtures?” first cited in Example 4.26, reported the following observations on X ⫽ voids filled with asphalt for 52 specimens of a certain type of hot-mix asphalt: 74.33 71.07 73.82 77.42 79.35 82.27 77.75 78.65 77.19 74.69 77.25 74.84 60.90 60.75 74.09 65.36 67.84 69.97 68.83 75.09 62.54 67.47 72.00 66.51 68.21 64.46 64.34 64.93 67.33 66.08 67.31 74.87 69.40 70.83 81.73 82.50 79.87 81.96 79.51 84.12 80.61 79.89 79.70 78.74 77.28 79.97 75.09 74.38 77.67 83.73 80.39 76.90 Let’s estimate the variance s 2 of the population distribution. A natural estimator is the sample variance: Minitab gave the following output from a request to display descriptive statistics: Variable Count Mean SE Mean StDev Variance Q1 Median Q3 VFAB 52 73.880 0.889 6.413 41.126 67.933 74.855 79.470 Thus the point estimate of the population variance is [alternatively, the computational formula for the numerator of s 2 gives A point estimate of the population standard deviation is then An alternative estimator results from using the divisor n rather than n ⫺ 1: We will shortly indicate why many statisticians prefer S 2 to this latter estimator. The cited article considered fitting four different distributions to the data: normal, log- normal, two-parameter Weibull, and three-parameter Weibull. Several different tech- niques were used to conclude that the two-parameter Weibull provided the best fit a normal probability plot of the data shows some deviation from a linear pattern. From Section 4.5, the variance of a Weibull random variable is where a and b are the shape and scale parameters of the distribution. The authors of the article used the method of maximum likelihood see Section 6.2 to estimate these parameters. The resulting estimates were . A sensible estimate of the population variance can now be obtained from substituting the esti- mates of the two parameters into the expression for s 2 ; the result is This latter estimate is obviously quite different from the sample variance. Its validity depends on the population distribution being Weibull, whereas the sample variance is a sensible way to estimate s 2 when there is uncertainty as to the specific form of the population distribution. ■ s ˆ 2 5 56.035. a ˆ 5 11.9731, bˆ 5 77.0153 s 2 5 b 2 5⌫1 1 2a 2 [⌫1 1 1a] 2 6 s ˆ 2 5 gX i 2X 2 2 n , estimate 5 2097.4124 52 5 40.335 s ˆ 5 s 5141.126 5 6.413. S xx 5 gx i 2 2 gx i 2 n 5 285,929.5964 2 3841.78 2 52 5 2097.4124]. s ˆ 2 5 s 2 5 g x i 2 x 2 52 2 1 5 41.126 s ˆ 2 5 S 2 5 g X i 2 X 2 2 n 2 1 Example 6.3 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. In the best of all possible worlds, we could find an estimator for which always. However, is a function of the sample X i ’s, so it is a random variable. For some samples, will yield a value larger than , whereas for other samples will underestimate . If we write then an accurate estimator would be one resulting in small estimation errors, so that estimated values will be near the true value. A sensible way to quantify the idea of being close to is to consider the squared error . For some samples, will be quite close to and the resulting squared error will be near 0. Other samples may give values of far from , corre- sponding to very large squared errors. An omnibus measure of accuracy is the expected or mean square error . If a first estimator has smaller MSE than does a second, it is natural to say that the first estimator is the better one. However, MSE will generally depend on the value of . What often happens is that one estimator will have a smaller MSE for some values of and a larger MSE for other values. Finding an estimator with the smallest MSE is typically not possible. One way out of this dilemma is to restrict attention just to estimators that have some specified desirable property and then find the best estimator in this restricted group. A popular property of this sort in the statistical community is unbiasedness. Unbiased Estimators Suppose we have two measuring instruments; one instrument has been accurately cal- ibrated, but the other systematically gives readings smaller than the true value being measured. When each instrument is used repeatedly on the same object, because of measurement error, the observed measurements will not be identical. However, the measurements produced by the first instrument will be distributed about the true value in such a way that on average this instrument measures what it purports to measure, so it is called an unbiased instrument. The second instrument yields observations that have a systematic error component or bias. u u MSE 5 E[uˆ 2 u 2 ] u uˆ u uˆ uˆ 2 u 2 u uˆ uˆ 5 u 1 error of estimation u uˆ u uˆ uˆ uˆ 5 u uˆ DEFINITION A point estimator is said to be an unbiased estimator of if for every possible value of . If is not unbiased, the difference is called the bias of . uˆ E uˆ 2 u uˆ u E uˆ 5 u u uˆ That is, is unbiased if its probability i.e., sampling distribution is always “cen- tered” at the true value of the parameter. Suppose is an unbiased estimator; then if ⫽ 100, the sampling distribution is centered at 100; if ⫽ 27.5, then the ˆ␪ sam- pling distribution is centered at 27.5, and so on. Figure 6.1 pictures the distributions of several biased and unbiased estimators. Note that “centered” here means that the expected value, not the median, of the distribution of ˆ␪ is equal to . u u uˆ u uˆ uˆ ␪ ␪ ␪ 1 Bias of ␪ 1 Bias of pdf of ␪ 2 ˆ pdf of ␪ 1 ˆ ˆ pdf of ␪ 2 ˆ pdf of ␪ 1 ˆ ˆ Figure 6.1 The pdf’s of a biased estimator and an unbiased estimator for a parameter u uˆ 2 uˆ 1 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. It may seem as though it is necessary to know the value of in which case estimation is unnecessary to see whether is unbiased. This is not usually the case, though, because unbiasedness is a general property of the estimator’s sampling distribution—where it is centered—which is typically not dependent on any particular parameter value. In Example 6.1, the sample proportion Xn was used as an estimator of p, where X, the number of sample successes, had a binomial distribution with parame- ters n and p. Thus E pˆ 5 E a X n b 5 1 n EX 5 1 n np 5 p uˆ u When X is a binomial rv with parameters n and p, the sample proportion ⫽ X n is an unbiased estimator of p. pˆ No matter what the true value of p is, the distribution of the estimator will be cen- tered at the true value. Suppose that X, the reaction time to a certain stimulus, has a uniform distribution on the interval from 0 to an unknown upper limit so the density function of X is rectan- gular in shape with height 1 for 0 ⱕ x ⱕ . It is desired to estimate on the basis of a random sample X 1 , X 2 , . . . , X n of reaction times. Since is the largest possible time in the entire population of reaction times, consider as a first estimator the largest sample reaction time: . If n ⫽ 5 and x 1 ⫽ 4.2, x 2 ⫽ 1.7, uˆ 1 5 max X 1 , c , X n u u u u u pˆ PROPOSITION x 3 ⫽ 2.4, x 4 ⫽ 3.9, and x 5 ⫽ 1.3, the point estimate of is 4.2, 1.7, 2.4, . Unbiasedness implies that some samples will yield estimates that exceed and other samples will yield estimates smaller than —otherwise could not possibly be the center balance point of ’s distribution. However, our proposed estimator will never overestimate the largest sample value cannot exceed the largest population value and will underestimate unless the largest sample value equals . This intuitive argument shows that is a biased estimator. More precisely, it can be shown see Exercise 32 that The bias of is given by n n ⫹ 1 ⫺ ⫽ ⫺ n ⫹ 1, which approaches 0 as n gets large. It is easy to modify to obtain an unbiased estimator of . Consider the estimator Using this estimator on the data gives the estimate 654.2 ⫽ 5.04. The fact that n ⫹ 1n ⬎ 1 implies that will overestimate for some samples and underesti- mate it for others. The mean value of this estimator is u uˆ 2 uˆ 2 5 n 1 1 n max X 1 , c , X n u uˆ 1 u u u uˆ 1 E uˆ 1 5 n n 1 1 u , u asince n n 1 1 , 1 b uˆ 1 u u u uˆ 1 u u u 3.9, 1.3 5 4.2 uˆ 1 5 max u Example 6.4 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. If is used repeatedly on different samples to estimate , some estimates will be too large and others will be too small, but in the long run there will be no systematic ten- dency to underestimate or overestimate . ■ u u uˆ 2 5 n 1 1 n n n 1 1 u 5 u E uˆ 2 5 E c n 1 1 n maxX 1 , . . . , X n d 5 n 1 1 n E [maxX 1 , . . . , X n ] Principle of Unbiased Estimation When choosing among several different estimators of , select one that is unbiased. u According to this principle, the unbiased estimator in Example 6.4 should be preferred to the biased estimator 1 . Consider now the problem of estimating s 2 . uˆ uˆ 2 PROPOSITION Let X 1 , X 2 , . . . , X n be a random sample from a distribution with mean and variance s 2 . Then the estimator is unbiased for estimating s 2 . sˆ 2 5 S 2 5 g X i 2 X 2 2 n 2 1 m Proof For any rv Y, VY ⫽ EY 2 ⫺ [EY] 2 , so EY 2 ⫽ VY ⫹ [EY] 2 . Applying this to gives ■ The estimator that uses divisor n can be expressed as n ⫺ 1S 2 n, so This estimator is therefore not unbiased. The bias is n ⫺ 1s 2 n ⫺ s 2 ⫽ ⫺s 2 n. Because the bias is negative, the estimator with divisor n tends to underestimate s 2 , and this is why the divisor n ⫺ 1 is preferred by many statisticians though when n is large, the bias is small and there is little difference between the two. Unfortunately, the fact that S 2 is unbiased for estimating s 2 does not imply that S is unbiased for estimating s. Taking the square root messes up the property of E c n 2 1S 2 n d 5 n 2 1 n ES 2 5 n 2 1 n s 2 5 1 n 2 1 5ns 2 2 s 2 6 5 s 2 as desired 5 1 n 2 1 e ns 2 1 nm 2 2 1 n ns 2 2 1 n nm 2 f 5 1 n 2 1 e g s 2 1 m 2 2 1 n 5VgX i 1 [E gX i ] 2 6 f E S 2 5 1 n 2 1 e gEX i 2 2 1 n E[ gX i 2 ] f S 2 5 1 n 2 1 c g X 2 i 2 gX i 2 n d Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. If X 1 , X 2 , . . . , X n is a random sample from a distribution with mean , then is an unbiased estimator of . If in addition the distribution is continuous and symmetric, then and any trimmed mean are also unbiased estimators of . m X | m X m The fact that is unbiased is just a restatement of one of our rules of expected value: E ⫽ for every possible value of for discrete as well as continuous distribu- tions. The unbiasedness of the other estimators is more difficult to verify. The next example introduces another situation in which there are several un- biased estimators for a particular parameter. Under certain circumstances organic contaminants adhere readily to wafer surfaces and cause deterioration in semiconductor manufacturing devices. The paper “Ceramic Chemical Filter for Removal of Organic Contaminants” J. of the Institute of Environmental Sciences and Technology, 2003: 59–65 discussed a recently devel- oped alternative to conventional charcoal filters for removing organic airborne molec- ular contamination in cleanroom applications. One aspect of the investigation of filter performance involved studying how contaminant concentration in air related to concentration on a wafer surface after prolonged exposure. Consider the following representative data on x ⫽ DBP concentration in air and y ⫽ DBP concentration on a wafer surface after 4-hour exposure both in gm 3 , where DBP ⫽ dibutyl phthalate. Obs. i: 1 2 3 4 5 6 x: .8 1.3 1.5 3.0 11.6 26.6 y: .6 1.1 4.5 3.5 14.4 29.1 The authors comment that “DBP adhesion on the wafer surface was roughly propor- tional to the DBP concentration in air.” Figure 6.2 shows a plot of y versus x—i.e., of the x, y pairs. m m m X X unbiasedness the expected value of the square root is not the square root of the expected value. Fortunately, the bias of S is small unless n is quite small. There are other good reasons to use S as an estimator, especially when the population distribu- tion is normal. These will become more apparent when we discuss confidence inter- vals and hypothesis testing in the next several chapters. In Example 6.2, we proposed several different estimators for the mean of a normal distribution. If there were a unique unbiased estimator for , the estimation problem would be resolved by using that estimator. Unfortunately, this is not the case. m m PROPOSITION Example 6.5 5 10

15 20

25 30 30 25 20 15 10 5 Wafer DBP Air DBP Figure 6.2 Plot of the DBP data from Example 6.5 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. If y were exactly proportional to x, we would have y ⫽ bx for some value b, which says that the x, y points in the plot would lie exactly on a straight line with slope b passing through 0, 0. But this is only approximately the case. So we now assume that for any fixed x, wafer DBP is a random variable Y having mean value bx . That is, we postulate that the mean value of Y is related to x by a line passing through 0, 0 but that the observed value of Y will typically deviate from this line this is referred to in the statistical literature as “regression through the origin”. We now wish to estimate the slope parameter b. Consider the following three estimators: The resulting estimates based on the given data are 1.3497, 1.1875, and 1.1222, respectively. So the estimate definitely depends on which estimator is used. If one of these three estimators were unbiased and the other two were biased, there would be a good case for using the unbiased one. But all three are unbiased; the argument relies on the fact that each one is a linear function of the Y i ’s we are assuming here that the x i ’s are fixed, not random: ■ In both the foregoing example and the situation involving estimating a normal pop- ulation mean, the principle of unbiasedness preferring an unbiased estimator to a biased one cannot be invoked to select an estimator. What we now need is a crite- rion for choosing among unbiased estimators. Estimators with Minimum Variance Suppose and are two estimators of that are both unbiased. Then, although the distribution of each estimator is centered at the true value of , the spreads of the dis- tributions about the true value may be different. u u uˆ 2 uˆ 1 E a gx i Y i gx 2 i b 5 1 gx i 2 E Qgx i Y i R 5 1 gx 2 i Qgx i bx i R 5 1 gx i 2 b Qgx i 2 R 5 b E a gY i gx i b 5 1 gx i E QgY i R 5 1 gx i Qgbx i R 5 1 gx i b Qgx i R 5 b E a 1 n g Y i x i b 5 1 n g E Y i x i 5 1 n g bx i x i 5 1 n gb 5 nb n 5 b [1: bˆ 5 1 n g Y i x i [2: bˆ 5 g Y i gx i [3: bˆ 5 g x i Y i gx i 2 Principle of Minimum Variance Unbiased Estimation Among all estimators of that are unbiased, choose the one that has minimum variance. The resulting is called the minimum variance unbiased estima- tor MVUE of . u uˆ u Figure 6.3 pictures the pdf’s of two unbiased estimators, with having smaller variance than . Then is more likely than to produce an estimate close to the true . The MVUE is, in a certain sense, the most likely among all unbiased estimators to produce an estimate close to the true . u u uˆ 2 uˆ 1 uˆ 2 uˆ 1 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. pdf of 2 ␪ ␪ pdf of 1 ␪ Figure 6.3 Graphs of the pdf’s of two different unbiased estimators In Example 6.5, suppose each Y i is normally distributed with mean bx i and variance s 2 the assumption of constant variance. Then it can be shown that the third estima- tor not only has smaller variance than either of the other two unbi- ased estimators, but in fact is the MVUE—it has smaller variance than any other unbiased estimator of b. We argued in Example 6.4 that when X 1 , . . . , X n is a random sample from a uniform distribution on [0, ], the estimator is unbiased for u we previously denoted this estimator by . This is not the only unbiased estimator of u. The expected value of a uniformly distributed rv is just the midpoint of the interval of positive density, so EX i ⫽ u2. This implies that E ⫽ u 2, from which E2 ⫽ . That is, the estimator is unbiased for . If X is uniformly distributed on the interval from A to B, then VX ⫽ s 2 ⫽ B ⫺ A 2 12. Thus, in our situation, VX i ⫽ u 2 12, V ⫽ s 2 n ⫽ 2 12n, and The results of Exercise 32 can be used to show that . The estimator has smaller variance than does if 3n⬍ nn ⫹ 2—that is, if 0 ⬍ n 2 ⫺ n ⫽ n n ⫺ 1. As long as n ⬎ 1, , so is a better estimator than . More advanced methods can be used to show that is the MVUE of u—every other unbiased estimator of u has variance that exceeds u 2 [nn ⫹ 2]. ■ One of the triumphs of mathematical statistics has been the development of methodology for identifying the MVUE in a wide variety of situations. The most important result of this type for our purposes concerns estimating the mean of a normal distribution. m uˆ 1 uˆ 2 uˆ 1 V uˆ 1 , Vuˆ 2 uˆ 2 uˆ 1 V uˆ 1 5 u 2 [nn 1 2] V uˆ 2 5 V2X 5 4VX 5 u 2 3n. u X u uˆ 2 5 2X u X X uˆ 2 uˆ 1 5 n 1 1 n max X 1 , c , X n u bˆ 5 gx i Y i gx i 2 THEOREM Let X 1 , . . . , X n be a random sample from a normal distribution with parame- ters and s . Then the estimator is the MVUE for . m X m ˆ 5 m Whenever we are convinced that the population being sampled is normal, the theo- rem says that 苶 x should be used to estimate . In Example 6.2, then, our estimate would be 苶 x ⫽ 27.793. In some situations, it is possible to obtain an estimator with small bias that would be preferred to the best unbiased estimator. This is illustrated in Figure 6.4. However, MVUEs are often easier to obtain than the type of biased estimator whose distribution is pictured. m Example 6.6 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. pdf of 2 , the MVUE ␪ ␪ pdf of 1 , a biased estimator ␪ Figure 6.4 A biased estimator that is preferable to the MVUE Some Complications The last theorem does not say that in estimating a population mean , the estimator should be used irrespective of the distribution being sampled. Suppose we wish to estimate the thermal conductivity of a certain material. Using standard measurement techniques, we will obtain a random sample X 1 , . . . , X n of n thermal conductivity measurements. Let’s assume that the population distribution is a member of one of the following three families: 6.1 6.2 6.3 The pdf 6.1 is the normal distribution, 6.2 is called the Cauchy distribution, and 6.3 is a uniform distribution. All three distributions are symmetric about , and in fact the Cauchy distribution is bell-shaped but with much heavier tails more proba- bility farther out than the normal curve. The uniform distribution has no tails. The four estimators for m considered earlier are , , e the average of the two extreme observations, and tr10 , a trimmed mean. The very important moral here is that the best estimator for depends cru- cially on which distribution is being sampled. In particular, 1 . If the random sample comes from a normal distribution, then is the best of the four estimators, since it has minimum variance among all unbiased estimators. 2 . If the random sample comes from a Cauchy distribution, then and e are terrible estimators for , whereas is quite good the MVUE is not known; is bad because it is very sensitive to outlying observations, and the heavy tails of the Cauchy distribution make a few such observations likely to appear in any sample. 3 . If the underlying distribution is uniform, the best estimator is e ; this estimator is greatly influenced by outlying observations, but the lack of tails makes such observations impossible. 4 . The trimmed mean is best in none of these three situations but works reason- ably well in all three . That is, tr10 does not suffer too much in comparison with the best procedure in any of the three situations. ■ More generally, recent research in statistics has established that when estimating a point of symmetry of a continuous probability distribution, a trimmed mean with trimming proportion 10 or 20 from each end of the sample produces reasonably m X X X X | m X X X m X X X | X m f x 5 u

1 2c

2c x 2 m c otherwise f x 5 1 p [1 1 x 2 m 2 ] 2` , x , ` f x 5 1 12ps 2 e 2 x2m 2 2s 2 2` , x , ` m X m Example 6.7 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. behaved estimates over a very wide range of possible models. For this reason, a trimmed mean with small trimming percentage is said to be a robust estimator. In some situations, the choice is not between two different estimators con- structed from the same sample, but instead between estimators based on two differ- ent experiments. Suppose a certain type of component has a lifetime distribution that is exponential with parameter l so that expected lifetime is ⫽ 1l. A sample of n such components is selected, and each is put into operation. If the experiment is continued until all n life- times, X 1 , . . . , X n , have been observed, then is an unbiased estimator of . In some experiments, though, the components are left in operation only until the time of the rth failure, where r ⬍ n. This procedure is referred to as censoring. Let Y 1 denote the time of the first failure the minimum lifetime among the n com- ponents, Y 2 denote the time at which the second failure occurs the second smallest lifetime, and so on. Since the experiment terminates at time Y r , the total accumu- lated lifetime at termination is We now demonstrate that is an unbiased estimator for . To do so, we need two properties of exponential variables: 1 . The memoryless property see Section 4.4, which says that at any time point, remaining lifetime has the same exponential distribution as original lifetime. 2 . When X 1 , . . . , X k are independent, each exponentially distributed with parame- ter l, minX 1 , . . . , X k , is exponential with parameter kl. Since all n components last until Y 1 , n ⫺ 1 last an additional Y 2 ⫺ Y 1 , n ⫺ 2 an addi- tional Y 3 ⫺ Y 2 amount of time, and so on, another expression for T r is T r ⫽ nY 1 ⫹ n ⫺ 1Y 2 ⫺ Y 1 ⫹ n ⫺ 2Y 3 ⫺ Y 2 ⫹ . . . ⫹ n ⫺ r ⫹ 1Y r ⫺ Y r ⫺ 1 But Y 1 is the minimum of n exponential variables, so EY 1 ⫽ 1nl. Similarly, Y 2 ⫺ Y 1 is the smallest of the n ⫺ 1 remaining lifetimes, each exponential with parameter l by the memoryless property, so EY 2 ⫺ Y 1 ⫽ 1[n ⫺ 1l]. Continuing, EY i ⫹ 1 ⫺ Y i ⫽ 1[n ⫺ il], so E T r ⫽ nEY 1 ⫹ n ⫺ 1EY 2 ⫺ Y 1 ⫹ . . . ⫹ n ⫺ r ⫹ 1EY r ⫺ Y r ⫺ 1 Therefore, ET r r ⫽ 1rET r ⫽ 1r ⭈ rl ⫽ 1l ⫽ m as claimed. As an example, suppose 20 components are tested and r ⫽ 10. Then if the first ten failure times are 11, 15, 29, 33, 35, 40, 47, 55, 58, and 72, the estimate of is The advantage of the experiment with censoring is that it terminates more quickly than the uncensored experiment. However, it can be shown that VT r r ⫽ 1l 2 r , which is larger than 1l 2 n , the variance of in the uncensored experiment. ■ X m ˆ 5 11 1 15 1 c 1 72 1 1072 10 5 111.5 m 5 r l 5 n 1 nl 1 n 2 1 1 n 2 1l 1 c 1 n 2 r 1 1 1 n 2 r 1 1l m m ˆ 5 T r r T r 5 g r i5 1 Y i 1 n 2 rY r m X m Example 6.8 Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Reporting a Point Estimate: The Standard Error Besides reporting the value of a point estimate, some indication of its precision should be given. The usual measure of precision is the standard error of the estimator used. DEFINITION The standard error of an estimator is its standard deviation . It is the magnitude of a typical or representative deviation between an estimate and the value of . If the standard error itself involves unknown parameters whose values can be estimated, substitution of these estimates into yields the estimated standard error estimated standard deviation of the estimator. The estimated standard error can be denoted either by the over s empha- sizes that is being estimated or by . s uˆ s uˆ ˆ s ˆ uˆ s ˆu u s ˆu 5 1V ˆu uˆ Assuming that breakdown voltage is normally distributed, is the best estima- tor of . If the value of s is known to be 1.5, the standard error of is . If, as is usually the case, the value of s is unknown, the estimate is substituted into to obtain the esti- mated standard error . ■ The standard error of ⫽ Xn is Since p and q ⫽ 1 ⫺ p are unknown else why estimate?, we substitute ⫽ xn pˆ s pˆ 5 2VXn 5 B V X n 2 5 B npq n 2 5 B pq n pˆ s ˆ X 2 5 s X 2 5 s 1n 5 1.462120 5 .327 s X 2 s ˆ 5 s 5 1.462 s X 2 5 s 1n 5 1.5 120 5 .335 X m ˆm 5 X Example 6.9 Example 6.2 continued Example 6.10 Example 6.1 continued The form of the estimator may be sufficiently complicated so that standard statistical theory cannot be applied to obtain an expression for . This is true, for s uˆ uˆ and ⫽ 1 ⫺ xn into yielding the estimated standard error . Alternatively, since the largest value of pq is attained when 1.6.425 5 .098 5 1pˆqˆn 5 sˆ pˆ s pˆ , qˆ p ⫽ q ⫽ .5, an upper bound on the standard error is ■ 114n 5 .10. When the point estimator has approximately a normal distribution, which will often be the case when n is large, then we can be reasonably confident that the true value of lies within approximately 2 standard errors standard deviations of . Thus if a sample of n ⫽ 36 component lifetimes gives and s ⫽ 3.60, then , so within 2 estimated standard errors, translates to the interval 28.50 ⫾ 2.60 ⫽ 27.30, 29.70. If is not necessarily approximately normal but is unbiased, then it can be shown that the estimate will deviate from by as much as 4 standard errors at most 6 of the time. We would then expect the true value to lie within 4 standard errors of and this is a very conservative statement, since it applies to any unbiased . Summarizing, the standard error tells us roughly within what distance of we can expect the true value of to lie. u uˆ uˆ uˆ u uˆ m ˆ s 1n 5 .60 m ˆ 5 x 5 28.50 uˆ u uˆ example, in the case ⫽ s, ; the standard deviation of the statistic S, s S , cannot in general be determined. In recent years, a new computer-intensive method called the bootstrap has been introduced to address this problem. Suppose that the population pdf is f x; , a member of a particular parametric family, and that data x 1 , x 2 , . . . , x n gives . We now use the computer to obtain “bootstrap samples” from the pdf fx; 21.7, and for each sample we calculate a “bootstrap estimate” : uˆ uˆ 5 21.7 u uˆ 5 S u Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. B ⫽ 100 or 200 is often used. Now let , the sample mean of the bootstrap estimates. The bootstrap estimate of ’s standard error is now just the sample stan- dard deviation of the : In the bootstrap literature, B is often used in place of B ⫺ 1; for typical values of B, there is usually little difference between the resulting estimates. A theoretical model suggests that X, the time to breakdown of an insulating fluid between electrodes at a particular voltage, has fx; l ⫽ le ⫺lx , an exponential distri- bution. A random sample of n ⫽ 10 breakdown times min gives the following data: 41.53 18.73 2.99 30.34 12.33 117.52 73.02 223.63 4.00 26.78 Since EX ⫽ 1l, E ⫽ 1l, so a reasonable estimate of l is . We then used a statistical computer package to obtain B ⫽ 100 bootstrap samples, each of size 10, from fx; .018153. The first such sample was 41.00, 109.70, 16.78, 6.31, 6.76, 5.62, 60.96, 78.81, 192.25, 27.61, from which and . The average of the 100 bootstrap esti- mates is , and the sample standard deviation of these 100 estimates is , the bootstrap estimate of ’s standard error. A histogram of the ’s was somewhat positively skewed, suggesting that the sampling distribution of also has this property. ■ Sometimes an investigator wishes to estimate a population characteristic without assuming that the population distribution belongs to a particular parametric family. An instance of this occurred in Example 6.7, where a 10 trimmed mean was proposed for estimating a symmetric population distribution’s center . The data of Example 6.2 gave , but now there is no assumed f x; , so how can we obtain a boot- strap sample? The answer is to regard the sample itself as constituting the population the n ⫽ 20 observations in Example 6.2 and take B different samples, each of size n, with replacement from this population. The book by Bradley Efron and Robert Tibshirani or the one by John Rice listed in the chapter bibliography provides more information. u uˆ 5 x tr10 5 27.838 u ˆl 100ˆl i ˆl s ˆ l 5 .0091 l 5 .02153 lˆ 1 5 154.58 5 .01832 gx i 5 545.8 5 .018153 lˆ 5 1 x 5 155.087 X S ˆu 5 B 1 B 2 1g ˆu i 2 u 2 uˆ i ’s ˆu u 5 ⌺uˆ i B B th bootstrap sample: x 1 , x 2 , c , x n ; estimate 5 uˆ B Second bootstrap sample: x 1 , x 2 , c , x n ; estimate 5 uˆ 2 First bootstrap sample: x 1 , x 2 , c , x n ; estimate 5 uˆ 1 Example 6.11 EXERCISES Section 6.1 1–19 1. The accompanying data on flexural strength MPa for con- crete beams of a certain type was introduced in Example 1.2. 5.9 7.2 7.3 6.3 8.1 6.8 7.0 7.6 6.8 6.5 7.0 6.3 7.9 9.0 8.2 8.7 7.8 9.7 7.4 7.7 9.7 7.8 7.7 11.6 11.3 11.8 10.7 a. Calculate a point estimate of the mean value of strength for the conceptual population of all beams manufactured in this fashion, and state which estimator you used. [Hint: ⌺x i ⫽ 219.8.] b. Calculate a point estimate of the strength value that sepa- rates the weakest 50 of all such beams from the strongest 50, and state which estimator you used. Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. c. Calculate and interpret a point estimate of the population standard deviation s. Which estimator did you use? [Hint: .] d. Calculate a point estimate of the proportion of all such beams whose flexural strength exceeds 10 MPa. [Hint: Think of an observation as a “success” if it exceeds 10.] e. Calculate a point estimate of the population coefficient of variation s , and state which estimator you used. 2. A sample of 20 students who had recently taken elementary statistics yielded the following information on the brand of calculator owned T ⫽ Texas Instruments, H ⫽ Hewlett Packard, C ⫽ Casio, S ⫽ Sharp: T T H T C T T S C H S S T H C T T T H T a. Estimate the true proportion of all such students who own a Texas Instruments calculator. b. Of the 10 students who owned a TI calculator, 4 had graphing calculators. Estimate the proportion of students who do not own a TI graphing calculator. 3. Consider the following sample of observations on coating thickness for low-viscosity paint “Achieving a Target Value for a Manufacturing Process: A Case Study,” J. of Quality Technology, 1992: 22–26: .83 .88 .88 1.04 1.09 1.12 1.29 1.31 1.48 1.49 1.59 1.62 1.65 1.71 1.76 1.83 Assume that the distribution of coating thickness is normal a normal probability plot strongly supports this assumption. a. Calculate a point estimate of the mean value of coating thickness, and state which estimator you used. b. Calculate a point estimate of the median of the coating thickness distribution, and state which estimator you used. c. Calculate a point estimate of the value that separates the largest 10 of all values in the thickness distribution from the remaining 90, and state which estimator you used. [Hint: Express what you are trying to estimate in terms of m and s.] d. Estimate PX ⬍ 1.5, i.e., the proportion of all thickness values less than 1.5. [Hint: If you knew the values of and s, you could calculate this probability. These values are not available, but they can be estimated.] e. What is the estimated standard error of the estimator that you used in part b? 4. The article from which the data in Exercise 1 was extracted also gave the accompanying strength observations for cylinders: 6.1 5.8 7.8 7.1 7.2 9.2 6.6 8.3 7.0 8.3 7.8 8.1 7.4 8.5 8.9 9.8 9.7 14.1 12.6 11.2 Prior to obtaining data, denote the beam strengths by X 1 , . . . , X m and the cylinder strengths by Y 1 , . . . , Y n . Suppose that the X i ’s constitute a random sample from a distribution with mean 1 and standard deviation s 1 and that the Y i ’s form a random sample independent of the X i ’s from another distribution with mean m 2 and standard deviation s 2 . m m m gx i 2 5 1860.94 a. Use rules of expected value to show that ⫺ is an unbi- ased estimator of 1 ⫺ 2 . Calculate the estimate for the given data. b. Use rules of variance from Chapter 5 to obtain an expres- sion for the variance and standard deviation standard error of the estimator in part a, and then compute the estimated standard error. c. Calculate a point estimate of the ratio s 1 s 2 of the two standard deviations. d. Suppose a single beam and a single cylinder are randomly selected. Calculate a point estimate of the variance of the dif- ference X ⫺ Y between beam strength and cylinder strength. 5. As an example of a situation in which several different statis- tics could reasonably be used to calculate a point estimate, consider a population of N invoices. Associated with each invoice is its “book value,” the recorded amount of that invoice. Let T denote the total book value, a known amount. Some of these book values are erroneous. An audit will be carried out by randomly selecting n invoices and determining the audited correct value for each one. Suppose that the sample gives the following results in dollars. m m Y X Invoice 1 2 3 4 5 Book value 300 720 526 200 127 Audited value 300 520 526 200 157 Error 200 ᎐ 30 Let ⫽ sample mean book value ⫽ sample mean audited value ⫽ sample mean error Propose three different statistics for estimating the total audited i.e., correct value—one involving just N and , another involving T, N, and , and the last involving T and . If N ⫽ 5000 and T ⫽ 1,761,300, calculate the three corresponding point estimates. The article “Statistical Models and Analysis in Auditing,” Statistical Science, 1989: 2–33 discusses properties of these estimators. 6. Consider the accompanying observations on stream flow 1000s of acre-feet recorded at a station in Colorado for the period April 1–August 31 over a 31-year span from an arti- cle in the 1974 volume of Water Resources Research. 127.96 210.07 203.24 108.91 178.21 285.37 100.85 89.59 185.36 126.94 200.19 66.24 247.11 299.87 109.64 125.86 114.79 109.11 330.33 85.54 117.64 302.74 280.55 145.11 95.36 204.91 311.13 150.58 262.09 477.08 94.33 Y X D X D X Y Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. An appropriate probability plot supports the use of the log- normal distribution see Section 4.5 as a reasonable model for stream flow. a. Estimate the parameters of the distribution. [Hint: Remember that X has a lognormal distribution with parameters and s 2 if lnX is normally distributed with mean and variance s 2 .] b. Use the estimates of part a to calculate an estimate of the expected value of stream flow. [Hint: What is EX?]

7. a.

A random sample of 10 houses in a particular area, each of which is heated with natural gas, is selected and the amount of gas therms used during the month of January is determined for each house. The resulting observations are 103, 156, 118, 89, 125, 147, 122, 109, 138, 99. Let denote the average gas usage during January by all houses in this area. Compute a point estimate of . b. Suppose there are 10,000 houses in this area that use nat- ural gas for heating. Let t denote the total amount of gas used by all of these houses during January. Estimate t using the data of part a. What estimator did you use in computing your estimate? c. Use the data in part a to estimate p, the proportion of all houses that used at least 100 therms. d. Give a point estimate of the population median usage the middle value in the population of all houses based on the sample of part a. What estimator did you use? 8. In a random sample of 80 components of a certain type, 12 are found to be defective. a. Give a point estimate of the proportion of all such compo- nents that are not defective. b. A system is to be constructed by randomly selecting two of these components and connecting them in series, as shown here. m m m m a. Find an unbiased estimator of m and compute the estimate for the data. [Hint: EX ⫽ m for X Poisson, so E ⫽ ?] b. What is the standard deviation standard error of your estimator? Compute the estimated standard error. [Hint: for X Poisson.] 10. Using a long rod that has length , you are going to lay out a square plot in which the length of each side is . Thus the area of the plot will be 2 . However, you do not know the value of , so you decide to make n independent measure- ments X 1 , X 2 , . . . , X n of the length. Assume that each X i has mean unbiased measurements and variance s 2 . a. Show that is not an unbiased estimator for m 2 . [Hint: For any rv Y, EY 2 ⫽ VY ⫹ [EY] 2 . Apply this with Y ⫽ .] b. For what value of k is the estimator ⫺ kS 2 unbiased for 2 ? [Hint: Compute E ⫺ kS 2 .] 11. Of n 1 randomly selected male smokers, X 1 smoked filter cig- arettes, whereas of n 2 randomly selected female smokers, X 2 smoked filter cigarettes. Let p 1 and p 2 denote the probabili- ties that a randomly selected male and female, respectively, smoke filter cigarettes. a. Show that X 1 n 1 ⫺ X 2 n 2 is an unbiased estimator for p 1 ⫺ p 2 . [Hint: EX i ⫽ n i p i for i ⫽ 1, 2.] b. What is the standard error of the estimator in part a? c. How would you use the observed values x 1 and x 2 to esti- mate the standard error of your estimator? d. If n 1 ⫽ n 2 ⫽ 200, x 1 ⫽ 127, and x 2 ⫽ 176, use the esti- mator of part a to obtain an estimate of p 1 ⫺ p 2 . e. Use the result of part c and the data of part d to esti- mate the standard error of the estimator. 12. Suppose a certain type of fertilizer has an expected yield per acre of 1 with variance s 2 , whereas the expected yield for a second type of fertilizer is 2 with the same variance s 2 . Let and denote the sample variances of yields based on sample sizes n 1 and n 2 , respectively, of the two fertilizers. Show that the pooled combined estimator is an unbiased estimator of s 2 . 13. Consider a random sample X 1 , . . . , X n from the pdf f x; ⫽ .51 ⫹ x ⫺ 1 ⱕ x ⱕ 1 where ⫺1 ⱕ ⱕ 1 this distribution arises in particle physics. Show that is an unbiased estimator of . [Hint: First determine ⫽ EX ⫽ E .] 14. A sample of n captured Pandemonium jet fighters results in serial numbers x 1 , x 2 , x 3 , . . . , x n . The CIA knows that the air- craft were numbered consecutively at the factory starting with a and ending with b, so that the total number of planes manu- factured is b ⫺ a ⫹ 1 e.g., if a ⫽ 17 and b ⫽ 29, then 29 ⫺ 17 ⫹ 1 ⫽ 13 planes having serial numbers 17, 18, 19, . . . , 28, 29 were manufactured. However, the CIA does not know the values of a or b. A CIA statistician suggests using the X m u uˆ 5 3X u u u sˆ 2 5 n 1 2 1S 1 2 1 n 2 2 1S 2 2 n 1 1 n 2 2 2 S 2 2 S 1 2 m m X 2 m X 2 X X 2 m m m m m s X 2 5 m X The series connection implies that the system will function if and only if neither component is defective i.e., both com- ponents work properly. Estimate the proportion of all such systems that work properly. [Hint: If p denotes the probabil- ity that a component works properly, how can Psystem works be expressed in terms of p?] 9. Each of 150 newly manufactured items is examined and the number of scratches per item is recorded the items are sup- posed to be free of scratches, yielding the following data: Number of scratches per item 1 2 3 4 5 6 7 Observed frequency 18 37 42 30 13 7 2 1 Let X ⫽ the number of scratches on a randomly chosen item, and assume that X has a Poisson distribution with parameter m. Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook andor eChapters. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. a.