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Related Experiment Video

Updated: Jun 4, 2026

Problem-Solving Before Instruction (PS-I): A Protocol for Assessment and Intervention in Students with Different Abilities

Problem-Solving Before Instruction (PS-I): A Protocol for Assessment and Intervention in Students with Different Abilities

Published on: September 11, 2021

How consistent are class size effects?

Spyros Konstantopoulos¹

¹Program in Measurement and Quantitative Methods, 450 Erickson Hall, College of Education, Michigan State University, East Lansing, MI 48824, USA. spyros@msu.edu

Evaluation Review

|March 3, 2011

Summary

Small class sizes benefit students in many schools, but effects vary significantly by school. This study analyzed Project STAR data, finding inconsistent small class impacts across different schools and grades.

Area of Science:

Education
Educational Psychology
Class Size Research

Background:

Previous research primarily focused on average achievement differences between small and large classes.
The impact of class size on student achievement can vary significantly across different school contexts.

Purpose of the Study:

To examine the distribution and inconsistency of small class effects at the school level.
To investigate the school-specific variation in the impact of small classes on student achievement.

Main Methods:

Utilized data from Project STAR, a large-scale educational study.
Estimated school-specific small class effects for mathematics and reading scores (K-3).
Calculated an overall weighted average of school-specific estimates.

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Main Results:

A significant proportion of schools showed positive small class effects.
However, some schools exhibited neutral or negative effects of small classes.
Small class effects were inconsistent across schools and grades, indicating a class size by school interaction.

Conclusions:

The benefits of small classes are not uniform and depend on the specific school environment.
There is substantial variability in how small class sizes influence student academic performance.
Further research should explore the factors contributing to the school-level heterogeneity in small class effects.