Supporting Research - Class Size

Project STAR

Tennessee's Project STAR (Students-Teacher Achievement Ratio) was designed to determine the short- and long-term effects of small class size in grades K-3 on student performance.

Phase 1: 1985-1989. K-3 classes of 13-17 students were compared with classes of 22-26 students. Findings:

  • Students in smaller classes substantially outperformed students in larger classes on both standardized and curriculum-based tests. This was true for white and minority students as well as inner city, suburban, and rural schools.
  • In smaller classes, fewer students were retained in-grade, and there was earlier identification of struggling students.

Phase 2: Lasting Benefits, 1989. This study began as a follow-up to the STAR study to examine if the effects of smaller class sizes stayed with students once they returned to larger classes. Findings:

In fourth grade, students from smaller classes still outperformed students from larger classes in all subjects and were better behaved.

Phase 3: Project Challenge, 1990. Based on these findings, Tennessee began phasing in smaller classes in grades K-3 in the poorest districts. Findings:

  • These districts moved from near the bottom of school district performance in Tennessee to near the middle in reading and mathematics for second grade.
  • In-grade retention was reduced.

1999 Update
Researchers reported that the effects of small class sizes in grades K-3 lasted all the way through high school. Students from small classes are:

  • More likely to graduate from high school on schedule and less likely to drop out;
  • More likely to have enrolled in honors classes and to graduate in the top 10 percent of their class; and
  • More likely to take SAT or ACT exams, indicating that they plan to go on to college. Further, the black-white achievement gap is reduced by 56 percent for black students who began school in small classes.

Researchers also found that students in small classes in grades K-3 were between six and 13 months ahead of their regular-class peers in math, reading, and science in each of grades 4, 6, and 8. Researchers reported that for the benefits to be sustained through later grades, at least three years in a small class are necessary. In addition, the benefits of having been in a small class in the primary years increase from grade to grade.

2010 Update
Researchers from Amherst College attempted to differentiate which kinds of schools and students benefit the most from smaller class sizes. Findings:

  • The researchers found that all students in high-poverty schools benefit from reduced class sizes, with high-achieving students benefiting the most.
  • Researchers also found no evidence of changes in teacher behavior or pedagogical practice when class sizes are reduced, indicating that “student differences account for the positive relationship between achievement and the benefit of smaller classes.”


Where We Stand: A STAR Story (1999)

Where We Stand: Class Size Revisited (1995)

Disruption, Achievement and the Heterogeneous Benefits of Smaller Classes, Graham McKee, Katherine Sims and Steven Rivkin, NBER Working Paper No. 15812, JEL No. I20,I21 (March 2010)

Would Smaller Classes Help Close the Black-White Achievement Gap?, Alan B. Krueger and Diane M. Whitmore, Bridging the Achievement Gap (2002)

The Effect of Attending a Small Class in the Early Grades on College-test Taking and Middle School Test Results: Evidence from Project STAR, Alan B. Krueger and Diane M. Whitmore, Economic Journal (2001)

Student Achievement Guarantee in Education (SAGE) Program

SAGE began in Wisconsin in 1996 by phasing in class size reduction in grades K-3 in school districts serving high-poverty students. The aim is to achieve a student/teacher ratio of 15:1.


  • SAGE first-, second-, and third-grade students performed consistently better than comparison students in mathematics, reading, and language arts on the Comprehensive Test of Basic Skills.
  • First- and third-grade, African-American SAGE students gained significantly more than SAGE white students, closing the achievement gap. The achievement gap widened for non-SAGE students.
  • Teachers reported more individualization in their instruction due to fewer discipline problems; being more knowledgeable about each student; and having more enthusiasm for teaching.

It should be noted that while SAGE findings are consistent with Project STAR findings, SAGE schools were also required to implement a rigorous academic curriculum, provide before and after-school activities, and implement professional development programs and accountability plans.


Class Size Reduction in Wisconsin: A Fresh Look at the Data, Education Policy Studies Laboratory (Sept. 2003)

The 1999-2000 Evaluation Results of the Student Achievement Guarantee in Education (SAGE) Program (Dec. 2000)

1998-99 Results of the Student Achievement Guarantee in Education (SAGE) Program(Dec. 1999)

Evaluating the SAGE Program: A Pilot Program in Targeted Pupil-Teacher Reduction in Wisconsin, Educational Evaluation and Policy Analysis (Summer 1999)

Reducing Class Size Leads to Individual Instruction, Educational Leadership (Sept. 1999)

Rouse Study

The Milwaukee Parental Choice Program is a publicly funded voucher program. In her study, Rouse compares the achievement of Milwaukee voucher students and students in three types of Milwaukee Public Schools: regular schools, magnet schools, and schools participating in the Preschool to Grade 5 Grant Program (P-5 schools). P-5 schools serve "predominantly minority and extremely disadvantaged" children and receive supplemental state funds that have enabled them to cut their pupil-teacher ratio to 17 to 1 on average. Findings:

  • Students in the P-5 (small class size) public schools made "substantially faster gains in reading" than those in the regular public schools, the public magnet schools, and the voucher schools.
  • Students in the P-5 (small class size) public schools made faster math gains than students in the regular public schools and the public magnet schools, and the same gains as the voucher schools.


Vouchers vs. Small Class Size (April 1998)

Schools and Student Achievement: More Evidence from the Milwaukee Parental Choice Program, Cecilia Rouse (Dec. 1997)

Smaller Classes, Not Vouchers, Increase Student Achievement, Alex Molnar (Jan. 1998)

Wenglinsky Study

Educational Testing Services (ETS) researcher Harold Wenglinsky compared 1992 fourth- and eighth-grade NAEP math results to class size and other policy initiatives. For purposes of the analysis, Wenglinsky defined small class size as fewer than 20 students.


  • Fourth- and eighth-graders in small classes performed better than those in large classes, even when taking into account demographics, resources, and cost of living.
  • Fourth-graders were one-third of a grade level ahead of their peers from large classes; eighth-graders were one-eighth of a grade level ahead.
  • The largest gains were found for inner-city students: fourth-graders were three-fourths of a grade level ahead of students in large classes.


When Money Matters, Harold Wenglinsky, ETS Policy Information Center (April 1997)

Class-Size Reduction (CSR)

During the mid-1990s, the California Legislature passed the Class-Size Reduction (CSR) incentive program, which provided funds to schools statewide that set class size to 20 students in grades K-3.

The $1 billion initiative was signed just six weeks before the start of the new school year. Even though the program was voluntary, there was significant pressure from parents and the press to reduce class size. By the end of the first year, 88 percent of first-graders were in reduced classes along with 57 percent of second-graders.

Due to the program's design, it is unclear how much effect the initiative has had on raising student performance, resulting in some officials questioning the cost-effectiveness of the program. Although researchers are unable to attribute achievement gains directly to CSR, California's efforts to reduce class sizes provide important lessons for education officials trying to implement small classes in their states and/or districts. CSR researchers point to a variety of reasons for the inconclusive results:

  • CSR in California was associated with declines in teacher qualifications and an inequitable distribution of qualified teachers. CSR caused an immediate need for 20,000 new teachers in a very small period of time. To meet the demand, teacher certification requirements were "relaxed" resulting in an influx of un- and under-prepared new teachers. In addition, qualified veteran teachers were taking advantage of the new vacancies to leave less advantaged districts for more desirable positions in more affluent districts.
  • CSR was offered in every school instead of targeting funds to disadvantaged schools. This "one size fits all" approach with funding resulted in wealthier districts that already had smaller classes receiving an initial boon of funding, while overcrowded districts were forced to dip into their general funds to cover shortfalls.
  • Funding was inadequate. CSR provided $650 per student compared to the $2,000 per student provided for the SAGE program.
  • Serious problems associated with overcrowded schools were ignored.
  • No trial program was conducted to explore various class-size reduction options.
  • The adopted definition of small classes (20 students) directly contradicted prior evidence and experiences of other states.


What We Have Learned About Class Size Reduction in California, CSR Research Consortium (Aug. 2002)

Class Size Reduction in California, Evaluation Findings, 1998-99, CSR Research Consortium (1999)

Maimonides' Rule, Israel

Due to rabbinic scholar Mainmonides' interpretation of the Torah regarding education, Israel has capped class size at 40 since 1969. This means that a grade of 80 students will have two classes of 40 each, but a grade of 81 students will have class sizes of 27, 27 and 26. The wide and random difference in class size provides a natural experiment.


  • Students in smaller classes had higher test scores in math and reading after their second and third years in small classes. Their advantage over students in large classes was greater in the third year than the second, suggesting that learning gains are cumulative over time. Percentile gains were roughly the same as the Tennessee Project STAR experiment.
  • Researchers used data from third- to fifth-graders, demonstrating that small classes are beneficial beyond the earliest years.


Using Maimonides' Rule to Estimate the Effect of Class Size on Scholastic Achievement, Joshua D. Angrist and Victor Lavy, The Quarterly Journal of Economics (May 1999)

Institute of Education Class Size Study, University of London

Researchers randomly selected 11,386 students in the United Kingdom and followed them from the age of 4 to 7, recording demographic variables along with class size and test scores. Teachers were surveyed about how they spent classroom time, and researchers systematically observed selected classrooms by noting interactions every 10 seconds.


  • Students in small classes gained up to 14 percentile points on reading tests compared to their counterparts in large classes, with low-achieving students posting the biggest gains. On math tests, students of all achievement levels consistently scored 10 percentile points higher in small classes. This effect is roughly the same as the effect found in the Tennessee Project STAR experiment.
  • Students in larger classes were twice as likely to be off-task and spent more time talking to each other. Their teachers reported spending less time on instruction and providing feedback for individual students.
  • Students in small classes had almost 50 percent more one-on-one interaction with their teachers than students in large classes; they also received more immediate and positive feedback.


Crowd Control: An International Look at the Relationship between Class Size and Student Achievement, Martin R. West and Ludget Woessmann, Education Next (Summer 2003)

Are Class Size Differences Related to Pupils' Educational Progress and Classroom Processes? Findings from the Institute of Education Class Size Study of Children Aged 5-7 Years, Peter Blatchford, Paul Bassett, Harvey Goldstein and Clare Martin, British Educational Research Journal (2003)