Conduct a test of H : m

26°C 5°C Rat Blood Pressure Rat Blood Pressure 1 152 7 384 2 157 8 369 3 179 9 354 4 182 10 375 5 176 11 366 6 149 12 423 Env. 6.6 A pollution-control inspector suspected that a riverside community was releasing semi- treated sewage into a river and this, as a consequence, was changing the level of dissolved oxygen of the river. To check this, he drew 15 randomly selected specimens of river water at a location above the town and another 15 specimens below. The dissolved oxygen readings, in parts per mil- lion, are given in the accompanying table. Above Town 5.2 4.8 5.1 5.0 4.9 4.8 5.0 4.7 4.7 5.0 4.7 5.1 5.0 4.9 4.9 Below Town 4.2 4.4 4.7 4.9 4.6

4.8 4.9

4.6 5.1 4.3 5.5 4.7 4.9 4.8 4.7 Use the computer output shown here to answer the following questions. a. Do the data provide sufficient evidence to indicate a difference in mean oxygen content between locations above and below the town? Use a ⫽ .05. b. Was the pooled t test or the separate-variance t test used in the computer output? c. Do the required conditions to use the test in a appear to be valid for this study? Justify your answer. d. How large is the difference between the mean oxygen content above and below the town? Engin. 6.7 An industrial engineer conjectures that a major difference between successful and unsuccess- ful companies is the percentage of their manufactured products returned because of defectives. In a study to evaluate this conjecture, the engineer surveyed the quality control departments of 50 suc- cessful companies identified by the annual profit statement and 50 unsuccessful companies. The companies in the study all produced products of a similar nature and cost. The percentage of the total output returned by customers in the previous year is summarized in the following graphs and tables. Boxplots of above- and below-town specimens means are indicated by solid circles 5.5 5.0 4.5 4.0 Above-town specimens Below-town specimens Dissolved oxygen content Two-Sample T-Test and Confidence Interval Two-sample T for Above Town vs Below Town N Mean StDev SE Mean Ab ove To 15 4.92 0.157 0.042 Below To 15 4.74 0.320 0.084 95 CI for mu Above To mu Below To: 0.013, 0.378 T-Test mu Above To mu Below To vs not : T 1.95 P 0.065 DF 20