672 The Journal of Human Resources
standard deviations. This finding for math is fully consistent with earlier studies by Monk 1994 and Monk and King 1994 who find, using national survey data, that
teacher preparation in math has positive effects on student test scores in math. The only other subject for which certification matters is English, where once again the
estimated effects are large.
F. National Board Certification
North Carolina has been a leader in the national movement to have teachers certified by the National Board for Professional Teaching Standards NBPTS, and provides
incentives in the form of a 12 percent boost in pay for teachers to do so. Such certification, which requires teachers to put together a portfolio and to complete a
variety of exercises and activities designed to test their knowledge of material in their particular field, takes well over a year and is far more difficult to obtain than
state licensure.
Following other researchers, we test both for the signaling effect of Board Cer- tification and a human capital effect Harris and Sass 2007 and Goldhaber and
Anthony 2007. A positive signaling effect emerges from Table 4 in the form of the positive coefficient of 0.0219 on the variable denoted pre-certification. This variable
takes on the value 1 for any teacher who ultimately will become Board Certified. The second Board Certification variable takes on the value 1 in the year in which
the candidate for certification is going through the process, and the third variable indicates that the teacher is Board Certified. The finding that the coefficients on the
two latter variables are statistically significantly larger than the pre-certification co- efficient provides evidence of a positive human capital effect. That is, teachers ap-
pear to become better teachers as a result of the Board Certification process. No evidence of a positive human capital effect emerged from our prior research on
Board Certification at the elementary level.
G. Summary measure of the effects of credentials
We use these estimated coefficients to develop a summary measure of the effects of teacher credentials. Specifically, we compare the achievement effects of a teacher
with weak credentials, defined as one at the tenth percentile in the predicted distri- bution of student achievement, where the predictions are based on teacher credentials
alone, with those of a teacher with strong credentials, defined as one at the ninetieth percentile of the teacher distribution.
28
Based on the teachers in our sample, the difference in predicted student achievement between the two teachers is 0.23 stan-
dard deviations. Thus, by this metric a student with a weak teacher would be ex- pected to perform 0.23 standard deviations lower than if she had a teacher with
strong credentials. Though credentials may be bundled in various ways, it is clear from the estimated coefficients that novice or lateral entry teachers and those not
certified in the field they are teaching or in a related field are most likely to be at the bottom of the distribution. We return in the conclusion to the question of whether
this difference is large or small.
28. For this calculation the teachers are not weighted by the number of students they teach.
Clotfelter, Ladd, and Vigdor 673
VI. Achievement Effects of Teacher and Classroom Characteristics
