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February 1, 2002
Vol. 59
No. 5

Measuring Class Size: Let Me Count the Ways

Determining class size seems simple enough, but it is actually more difficult— and more important—than it appears.

Consider a classroom with 30 students and one teacher. The class size, or the number of students for whom the teacher is responsible and accountable on a daily basis, is 30. The pupil-teacher ratio (PTR), or the number of students at a site or in a classroom divided by the total number of educators who serve the site or classroom, is 30 to 1. If the classroom has 30 students and two teachers, then the class size is still 30—but the pupil-teacher ratio is now 15 to 1. If one teacher and several additional education professionals—for example, a speech therapist, a Reading Recovery teacher, and others—serve the same 30 students, the class size is still 30. What has changed? In each case, the pupil-teacher ratio has changed, but the classroom teacher is still responsible for a class of 30 students.
Class size involves organizing students for the delivery of instruction, whereas pupil-teacher ratio is an administrative statistic that helps account for the distribution of resources. The approximate difference between pupil-teacher ratio and class size in U.S. schools is 10. That is, if the school's pupil-teacher ratio is 17 to 1, then most teachers in the building will have, on average, 27 students per class (Achilles & Sharp, 1998).
Using the terms class size and pupil-teacher ratio interchangeably confounds how people think about class size and accomplish class size reduction. Discussions of class size and its effects have been clouded by such imprecise definitions, as well as by the substitution of pupil-teacher ratio results for class size outcomes and the use of pupil-teacher ratio data to challenge class size findings.

Pupil-Teacher Ratio Isn't Class Size

To implement legislation that mandates special help for students with disabilities, educators have provided such pupil-teacher ratio interventions as specialized teachers and teacher aides. In fact, according to the 1999 Digest of Education Statistics (National Center for Education Statistics, 2000), the elementary school pupil-teacher ratio has fallen from 30.2 in 1955 to 18.6 in 1997.
Unfortunately, initiatives that aim to reduce pupil-teacher ratio do not achieve the goal of class size reduction: a small learning community of students with a quality teacher. In a qualitative study, Ladson-Billings and Gomez (2001) reported that “students who receive services from a variety of professionals were more likely to be confused about to whom they were responsible” (p. 677).
Hong (2001), an elementary teacher, noted that most days [were] broken into shards of time . . . when certain students would be coming and going for various pullout programs. Consequently the curriculum had to be chopped into segments and compressed. (p. 712)
A yearlong observation study in grades K–2 (Achilles, Kiser-Kling, Aust, & Owen, 1995) revealed that a student might start a lesson, leave in the middle of that lesson, and return at the midpoint of the next lesson, thereby experiencing the beginning of reading and the end of math. Pulling students out of class was distracting for the rest of the class as well. By contrast, the researchers found that students in small classes spent more time on task, misbehaved less frequently, and had greater test-score gains.
Confusion about class size often results from studies that analyze data from large databases or surveys—especially upper-grades data, such as the scores of 11th grade students on the National Assessment of Educational Progress (NAEP). These large databases provide pupil-teacher ratio data. Schools and districts with many specialized education professionals to provide remedial services to students who typically struggle on standardized tests will have favorable pupil-teacher ratios. Because pupil-teacher ratio changes do not influence overall student performance much, however, people who use pupil-teacher ratio as a proxy for class size may conclude that class size does not make much of a difference for students. But what do studies of class size show?

Class Size Research

Early class size meta-analyses (Glass & Smith, 1978; Smith & Glass, 1979) and other studies laid the groundwork for Tennessee's Student Teacher Achievement Ratio (STAR) study, a statewide longitudinal education experiment to analyze the effects of class size reduction on 11,601 K–3 students from 1985 to 1989. Researchers randomly assigned students entering kindergarten in 1985 to either a small class (one teacher and 13–17 students), a regular class (one teacher and 22–25 students), or a regular class with one teacher and a full-time teacher's aide. The only manipulated variable was class size (or, in the case of the classrooms with aides, the pupil-teacher ratio).
Researchers determined that students who had been in small classes had higher levels of achievement than their peers in regular classes or in classes with aides. In addition, small class students' extra gains were greater the longer those students had been in small classes. That is, without continuous exposure to small classes in grades K–3, students will not experience substantive test-score gains in later years (Finn, Gerber, Achilles, & Boyd-Zaharias, 2001).
  • Early intervention—students start schooling in small classes in kindergarten or prekindergarten.
  • Duration—students are in small classes for at least three, and preferably four, years.
  • Intensity—students are in small classes every day, all day.
Recent reanalyses of the original Project STAR data using newer statistical models emphasized the importance of early intervention and duration. By grade 2, STAR students who had been in small classes for one, two, and three years were several months ahead of their large-class and aide-class peers in reading. By grade 8, STAR students with four years (K–3) of small classes were nearly one year ahead of their randomly assigned peers who had spent their early years in large or aide classes (Finn et al., 2001).
Researchers have found that small classes provide additional benefits for minority and low-income students, especially in high-poverty areas (Finn & Achilles, 1990; Robinson, 1990; Wenglinsky, 1997). Aside from reducing the test-score gap, small classes also reduce retention in grade and student discipline referrals (Achilles, Finn, & Bain, 1997/1998; Word et al., 1990).
When compared with randomly assigned peers in larger classes or larger classes with a full-time aide in grades K–3, the small class students had higher graduation rates and lower dropout rates in high school. And small classes also may result in improved teacher morale: Even upper-grade teachers enjoy the benefits of the early behavior and performance improvements that their students made in the small K–3 classes (Achilles, 1999; Boyd-Zaharias & Pate-Bain, 2000).
The differences found in the STAR students' early years persisted into high school and beyond. Krueger and Whitmore (1999) reported that “attending a small class reduced the black-white gap in the college-entrance-test-taking rate by 54 percent” (p. 2).

Lessons Learned

Pupil-teacher ratio and class size shouldn't be confused in terms of either operation or outcomes. School leaders should strive to carefully monitor and report actual class sizes and to use the difference between pupil-teacher ratio and class size as a basis for reallocating school personnel.
School leaders should consider the three principles of effective class size reduction—early intervention, duration, and intensity—when developing and implementing plans to reduce class sizes. Educators should first implement small classes in kindergarten and grade 1 and gradually expand the program to include other grades. They should keep students in small classes for as long as possible because students experience minimal effects after only one year in a small class. Finally, educators should design small classes as learning communities, free from the frantic disruption of pullouts.

Achilles, C. M. (1999). Let's put kids first finally: Getting class size right. Thousand Oaks, CA: Corwin Press.

Achilles, C. M., & Sharp, M. (1998, Fall). Solve your puzzles using class size and pupil-teacher ratio (PTR) differences. Catalyst for Change, 28(1), 5–10.

Achilles, C. M., Finn, J. D., & Bain, H. P. (1997/1998). Using class size to reduce the equity gap. Educational Leadership, 55(4), 40–43.

Achilles, C. M., Kiser-Kling, K., Aust, A., & Owen, J. (1995, April). A study of reduced class size in primary grades of a fully Chapter-1 eligible school: Success starts small (SSS). Paper presented at the American Educational Research Association, San Francisco. (ERIC Document Reproduction No. ED 419 288)

Boyd-Zaharias, J., & Pate-Bain, H. (2000). Early and new findings from Tennessee's Project STAR. In M. C. Wang & J. D. Finn (Eds.), How small classes help teachers do their best (pp. 65–98). Philadelphia: Temple University, Center for Research in Human Development in Education.

Finn, J. D., & Achilles, C. M. (1990). Answers and questions about class size: A statewide experiment. American Educational Research Journal, 27(3), 557–577.

Finn, J., Gerber, S. B., Achilles, C. M., & Boyd-Zaharias, J. (2001). The enduring effects of small classes. Teachers College Record, 103(2), 145–183.

Glass, G. V., & Smith, M. L. (1978). Meta-analysis of research on the relationship of class size and achievement. San Francisco: Far West Laboratory for Educational Research and Development.

Hong, L. K. (2001). Too many intrusions on instructional time. Phi Delta Kappan, 82(9), 712–714.

Krueger, A. B., & Whitmore, D. M. (1999, April). The effect of attending a small class in the early grades on college attendance plans (Executive Summary). Princeton, NJ: Princeton University.

Ladson-Billings, G., & Gomez, M. L. (2001). Just showing up: Supporting early literacy through teachers' professional communities. Phi Delta Kappan, 82(9), 675–680.

Snyder, T. D. (2000). Digest of education statistics, 1999. Washington, DC: U.S. Department of Education, National Center for Education Statistics.

Robinson, G. L. (1990). Synthesis of research on the effects of class size. Educational Leadership, 47(7), 80–90.

Smith, M. L., & Glass, G. V. (1979). Relationship of class-size to classroom processes, teacher satisfaction and pupil affect: A meta-analysis. San Francisco: Far West Laboratory for Educational Research and Development.

Wenglinsky, H. (1997). When money matters. Princeton, NJ: Educational Testing Service Policy Information Center.

Word, E., Johnston, J., Bain, H. P., Fulton, D., Zaharias, J., Lintz, N., Achilles, C. M., Folger, J., & Breda, C. (1990). Student/Teacher Achievement Ratio (STAR): Tennessee's K–3 class size study. Final report and final report summary. Nashville: Tennessee State Department of Education.

Helen Pate-Bain has been a contributor in Educational Leadership.

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