Effect Size Calculator

Between-Groups Comparison

Group 1 (Treatment)

Mean M₁

SD SD₁

Group 2 (Control)

Mean M₂

SD SD₂

Enter values for both groups to compute effect sizes.
Requires n ≥ 2 and SD > 0 for each group.

Morris (2008) d_ppc2

Uses pretest pooled SD as the standardizer and adjusts the SE for the pretest–posttest correlation.

Treatment Group

Pretest M

Posttest M

Pretest SD

Control Group

Pretest M

Posttest M

Pretest SD

Pre–Post Correlation (r)

Higher correlations → smaller SE → narrower confidence intervals. If unknown, r = .50 is a common conservative default.

Enter values for both groups to compute effect sizes.
Requires n ≥ 2, SD > 0, and −1 ≤ r ≤ 1.

Paired / Repeated Measures

For within-subjects or matched-pairs designs (e.g., same raters rating two groups, pre-post without a control).

Condition A / Time 1

Mean M₁

SD SD₁

n (pairs)

Condition B / Time 2

Mean M₂

SD SD₂

Correlation Between Measures (r)

If unknown, r = .50 is a conservative default. If you have the SD of the difference scores, you can enter those directly below instead.

Optional: Enter Difference Score SD Directly

SD_diff (overrides r)

Enter values for both conditions to compute effect sizes.
Requires n ≥ 2 and SD > 0. Provide either r or SD_diff.

References: Cohen, J. (1988). Statistical power analysis for the behavioral sciences. Hedges, L. V. (1981). Journal of Educational Statistics, 6(2). Morris, S. B. (2008). Organizational Research Methods, 11(2), 364–386. Cumming, G. (2012). Understanding the new statistics. Borenstein, M., et al. (2009). Introduction to meta-analysis.