Effect Size Calculators

In simple terms, a measure of effect size povides a standardized measure of the strength or magnitude of an effect. A statistical significance test tells us how confident we can be that there is an effect - for example, that hitting people over the head will decrease their ability to recall items on a list. A measure of effect size, such as Cohen's D, gives us a standardized way of assessing the magnitude of the effect.

In practice, you're only ever likely to calculate an effect size if you already know the effect is statistically significant (because there's no point in calculating the size of an effect, if there is no good reason to suppose there is any effect), and the particular way an effect size is calculated is related to the significance test performed.

Note: To calculate an effect size, you need to know the means and standard deviations of your groups. If you have raw data, you can calculate means here and standard deviations here.

Choose Your Effect Size Calculator

• Cohen's D for T-Test (Independent Samples)
• Cohen's D for Single Sample Z-Test