# Effect Size (Cohen's *d*) Calculator in Excel

## What is Effect Size and Cohen's *d*?

Effect Size measures the relationship between two variables (how large an effect is). The larger the effect size, the stronger the relationship between two variables.

Cohen's *d* measures the DIFFERENCE between two variables' means divided by the Standard Deviation. If two groups have similar standard deviations and have the same group size, use Cohen's *d*. Note: It can only be used with normal data distributions.

Cohen's *d* Effect Size categorization:

*d* = 0.2 | SMALL (0.2 means the difference between the two groups' means is less than 0.2 Standard Deviations)

*d* = 0.3 - 0.5 | MEDIUM

*d* = 0.8 + | LARGE

*NOTE: A d of 1 suggests the two groups differ by 1 Standard Deviation, while a d of 2 suggests 2 Standard Deviations, etc.*

### Where are these calculators located within QI Macros?

These values are calculated automatically and can be found in the following t-Test templates, located in the QI Macros > Stat Templates drop-down menu:

- Two-Sample Assuming Equal Variances
- Two-Sample Assuming Unequal Variances
- Paired Two Sample for Means

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