Effect size is a simple way of quantifying the difference between two groups that has many advantages over the use of tests of statistical significance alone.

Effect sizes facilitate the decision whether a clinically relevant effect is found, helps determining the sample size for future studies, and facilitates comparison between scientific studies.

The resulting effect size is called d_{Cohen} and it represents the difference between the groups in terms of their common standard deviation.

**Formula to calculate effect size.**

The standard deviation used here is the standard deviation of one of the groups.

**Example:**

Suppose you have the mean of the scores of grade 1 and 2 students as 25 and 20, calculate the effect size if the standard deviation of grade 2 is 2.

Therefore, the d_{Cohen} is 2.5.