Enter the mean and standard deviation of two separate groups of data. The calculator will display the Cohn’s D, also known as effect size, of the two data sets.

Cohen’s D Calculator

The following formula is used to calculate the effective size of two data sets.

Cd = (M2 – M1) ⁄ Sp

Sp= √((S12 + S22) ⁄ 2)

  • Where Cd is cohen’s D
  • M2 and M1 are the means
  • S1 and S2 are the standard deviations
  • Sp is the pooled standard deviation

Cohen’s D Definition

Cohen’s D is the proper effective size measure of two groups of data.

Cohen’s D Example

How to calculate Cohen’s D

  1. First, determine the means

    Calculate the means of each data set using a standard formula averages.

  2. Next, determine the standard deviations

    Calculate the standard deviations, s1 and s2, of each data set.

  3. Calculate the pooled standard deviation

    Using s1 and s2, calculate the pooled standard deviation of the set.

  4. Finally, calculate Cohen’s D

    Using the means from step 1 and the pooled standard deviation, calculate the Cohen’s d.


What is Cohen’s D?

Cohen’s D is the correct effective size measure of two groups of data with similar standard deviations and the same size.

What is standard deviation?

Standard deviation is the average variation from the mean of a data set.