# Effect Size Formula

To compare the two given observations we use effect size formula. To make some important decision and access other results from the comparison of the two sets of the data we use this method. Effect size formula is also used to predict and forecast possibilities by comparing the data sets. We first calculate the mean and then subtract them. Standard deviation is also calculated for both the observations and then we find the squares.

$\LARGE d \; = \; \frac{M_{1}-M_{2}}{\sqrt{\frac{S_{1}^{2}+S_{2}^{2}}{2}}}$

From the value “d” we can find the effect size coefficient from the following formula:

$\LARGE r \; = \; \frac{d}{\sqrt{d^{2}+4}}$

Where,
d = Cohen’s index
M1 = Mean of first observation
M2 = Mean of second observation
S1 = Standard deviation of first observation
S2 = Standard deviation of second observation
r = Effect-size coefficient.