Enter the mean of group 1 and group 2, and the pooled standard deviation into the calculator to determine the Effect Size Index.

## Effect Size Index Formula

The following formula is used to calculate the Effect Size Index, also known as Cohen’s d.

d = (M1 - M2) / SDpooled

Variables:

- d is the Effect Size Index (Cohen’s d)
- M1 is the mean of group 1
- M2 is the mean of group 2
- SDpooled is the pooled standard deviation

To calculate the Effect Size Index, subtract the mean of group 2 from the mean of group 1. Then, divide the result by the pooled standard deviation. The pooled standard deviation is calculated by taking the square root of the average of the squared standard deviations of the two groups.

## What is an Effect Size Index?

An Effect Size Index is a statistical tool used to measure the strength or magnitude of a relationship between variables in a study. It quantifies the size of the difference between groups, or the strength of the relationship between variables, providing a more meaningful measure than just a p-value from a hypothesis test. It allows researchers to understand not just if a treatment or condition has an effect, but the size of that effect, which can be crucial in fields like social sciences, education, and medicine.

## How to Calculate Effect Size Index?

The following steps outline how to calculate the Effect Size Index (Cohen’s d).

- First, determine the mean of group 1 (M1).
- Next, determine the mean of group 2 (M2).
- Next, determine the pooled standard deviation (SDpooled).
- Next, use the formula d = (M1 – M2) / SDpooled to calculate the Effect Size Index.
- Finally, check your answer with the calculator above.

**Example Problem : **

Use the following variables as an example problem to test your knowledge.

mean of group 1 (M1) = 10

mean of group 2 (M2) = 8

pooled standard deviation (SDpooled) = 2