Cohen's D Effect Size Calculator for Z-Test

For the single sample Z-test, Cohen's d is calculated by subtracting the population mean (before treatment) from the sample mean (after treatment), and then dividing the result by the population's standard deviation.

Cohen's d = (Msample - µpopulation) ⁄ σ

Interpreting Cohen's d

d = 0.2 Small effect - mean difference is 0.2 standard deviation
d = 0.5 Medium effect - mean difference is 0.5 standard deviation
d = 0.8 Large effect - mean difference is 0.8 standard deviation

Enter Your Values

Please enter your population mean (µ), sample mean (M) and population standard deviation (σ), and when you're ready, press the "Calculate" button.

Population mean (µ):
Sample mean (M):
Population SD (σ):


Please enter your values in the boxes above, and press the "Calculate" button.

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