The impact of teacher subject knowledge owithin-teacher within-student variation
Johannes Metzler a, Ludger Woessmann b,a University of Munich and Monitor Group, Germanyb University of Munich and Ifo Institute for Economic Research, CESifo and IZA, Poschingerstr. 5
2012 Elsevier B.V. All rights reserved.
ationalis cleadent leacterisand Riere is l
Journal of Development Economics 99 (2012) 486496
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Journal of Development Economics
j ourna l homepage: www.e lVirtually the only attribute that has been shown to be more frequently test scores in two academic subjects not only for each student, butalso for each teacher. This allows us to identify the impact of teachers'academic performance in a specic subject on students' academicperformance in the subject, while at the same time holding constantany student characteristics and any teacher characteristics that donot differ across subjects. We can observe whether the same studenttaught by the same teacher in two different academic subjects per-forms better in one of the subjects if the teacher's knowledge is rela-tively better in this subject. We develop a correlated random effectsmodel that generalizes conventional xed-effects models with xed ef-
We thank Sandy Black, Jere Behrman, David Card, Paul Glewwe, Eric Hanushek, PatrickKline,Michael Kremer, Javier Luque, Lant Pritchett, MartinWest, the editor Duncan Thomas,two anonymous referees, and seminar participants at UC Berkeley, Harvard University, theUniversity of Munich, the Ifo Institute, the Glasgow congress of the European EconomicAssociation, the CESifo Area Conference on Economics of Education, the IZA confer-ence on Cognitive and Non-Cognitive Skills, and the Frankfurt conference of the GermanEconomicAssociation for helpful discussion and comments. Special gratitude goes to Andrs
Burga Len of theMinisterio de Educacin del Per for provfor the teacher tests.Woessmann is thankful for the supporCampbell and Rita Ricardo-Campbell National Fellowship oUniversity. Support by the Pact for Research and Innovatiothe Regional Bureau for Latin America and the Caribbeanment Programme are also gratefully acknowledged. Corresponding author.
E-mail addresses: email@example.com (J. Metzler)(L. Woessmann).
URL: http://www.cesifo.de/woessmann (L. Woessm
0304-3878/$ see front matter 2012 Elsevier B.V. Aldoi:10.1016/j.jdeveco.2012.06.002ittle evidence that thosesalary decisions, namelycial for teacher quality.
venting problems from unobserved teacher traits and non-randomsorting of students to teachers.
We use a unique primary-school dataset from Peru that contains
characteristics most often used in hiring andteachers' education and experience, are cruPeru
One of the biggest puzzles in educteacher quality puzzle: while therequality is a key determinant of stuabout which specic observable charcount for this impact (e.g., Hanushek2005; Rockoff, 2004). In particular, thproduction today is ther evidence that teacherarning, little is knowntics of teachers can ac-vkin, 2010; Rivkin et al.,
signicantly correlated with student achievement is teachers' academicskills measured by scores on achievement tests (cf. Eide et al., 2004;Hanushek and Rivkin, 2006; Wayne and Youngs, 2003). The problemwith the latter evidence, however, is that issues of omitted variablesand non-random selection are very hard to address when estimatingcausal effects of teacher characteristics.1 In this paper, we estimate theimpact of teachers' academic skills on student achievement, circum-Student achievement
Teacher knowledgea b s t r a c ta r t i c l e i n f o
Article history:Received 18 October 2011Received in revised form 8 June 2012Accepted 9 June 2012
Teachers differ greatly in howaccount for this. We estimawithin-teacher within-studstudents and their teachersper-grade schools, we circumodel that identies fromone standard deviation in9% of a standard deviation idifferent from zero. Effectsiding uswith reliability statisticst and hospitality by theW.Glennf theHoover Institution, Stanfordn of the Leibniz Association andof the United Nations Develop-
l rights reserved.n student achievement: Evidence from
, 81679 Munich, Germany
uch they teach their students, but little is known about which teacher attributeshe causal effect of teacher subject knowledge on student achievement usingvariation, exploiting a unique Peruvian 6th-grade dataset that tested bothtwo subjects. Observing teachers teaching both subjects in one-classroom-ent omitted-variable and selection biases using a correlated random effectsfferences between the two subjects. After measurement-error correction,ject-specic teacher achievement increases student achievement by aboutath. Effects in reading are signicantly smaller and mostly not signicantlydepend on the teacher-student match in ability and gender.
sev ie r .com/ locate /devecfects for students, teachers, and subjects. Ourmodel identies the effectbased on within-teacher within-student variation and allows us to testthe overidentication restrictions implicit in the xed-effects models.
1 Recently, Lavy (2011) estimates the effect of teaching practices on studentachievement.
We can additionally restrict our analysis to small, mostly rural schoolswith atmost one teacher per grade, excluding any remaining possibilitythat parents may have chosen a specic teacher for their students inboth subjects based on the teacher's specic knowledge in one subject,or of bias from non-random classroom assignments more generally (cf.Rothstein, 2010).
We nd that teacher subject knowledge exerts a statistically andquantitatively signicant impact on student achievement. Oncecorrected for measurement error in teacher test scores, our resultssuggest that a one standard deviation increase in teacher test scoresraises student test scores by about 9% of a standard deviation inmath. Results in reading are signicantly smaller and indistinguish-
United States include Ehrenberg and Brewer (1995), Ferguson (1998),Ferguson and Ladd (1996), Hanushek (1971, 1992), Murnane andPhillips (1981), Rockoff et al. (2011), Rowan et al. (1997), and
487J. Metzler, L. Woessmann / Journal of Development Economics 99 (2012) 486496able from zero in most specications. We also nd evidence that theeffect depends on the ability and gender match between teachersand students, in that above-median students are not affected by var-iation in teacher subject knowledge when taught by below-medianteachers and that female students are not affected by variation inteacher subject knowledge when taught by male teachers. The aver-age math result means that if a student was moved, for example,from a teacher at the 5th percentile of the distribution of teachermath knowledge to a teacher at the median of this distribution, thestudent's math achievement would increase by 14.3% of a standarddeviation by the end of the school year. Thus, teacher subject knowl-edge is a relevant observable factor that is part of overall teacherquality.
Methodologically, our identication strategy for teacher effectsextends the within-student comparisons in two different subjectsproposed by Dee (2005, 2007) as a way of holding student characteris-tics constant.2 Because the teacher characteristics analyzed by Dee gender, race, and ethnicity do not varywithin teachers, that approachuses variation across different teachers, so that the identifying variationmay still be affected by selection and unobserved teacher charac-teristics. By contrast, our approach uses variation within individualteachers, a feature made possible by the fact that subject knowl-edge does vary within teachers. In our correlated random effectsmodel, we actually reject the overidentication restriction impliedin the xed-effects model that teacher knowledge effects are thesame across subjects. While in addition to teacher subject knowl-edge per se, the identied estimate may also capture the effect ofother features correlated with teacher subject knowledge such aspassion for the subject or experience in teaching the subject curricu-lum, robustness tests conrm that results are not driven by observedbetween-subject differences in instruction time, teachingmethods, andstudent motivation.
The vast literature on education production functions hints atteacher knowledge as one possibly the only factor quite consis-tently associated with growth in student achievement.3 In his earlyreview of the literature, Hanushek (1986, p. 1164) concluded thatthe closest thing to a consistent nding among the studies is thatsmarter teachers, ones who perform well on verbal ability tests, dobetter in the classroom although even on this account, the existingevidence is not overwhelming.4 Important studies estimating the as-sociation of teacher test scores with student achievement gains in the
2 Further examples using this between-subject method to identify effects of specicteacher attributes such as gender, ethnicity, credentials, teaching methods, instructiontime, and class size include Altinok and Kingdon (2012), Ammermller and Dolton(2006), Clotfelter et al. (2010), Dee and West (2011), Lavy (2010), and Schwerdt andWuppermann (2011).
3 The Coleman et al. (1966) report, which initiated this literature, rst found thatverbal skills of teachers were associated with better student achievement.
4 A decade later, based on 41 estimates of teacher test-score effects, Hanushek(1997, p. 144) nds that of all the explicit measures [of teachers and schools] that lendthemselves to tabulation, stronger teacher test scores are most consistently related tohigher student achievement. Eide et al. (