## Quartile Deviation for grouped data

Quartile deviation is half the difference between the thrid quartile $Q_3$ and the first quartile $Q_1$ of a frequency distribution. It is also known as **Semi-interquartile range**.

## Formula

The quartile deviation is given by

`$QD=\dfrac{Q_3-Q_1}{2}$`

and the coefficient of quartile deviation is given by

### Coeff. of `$QD=\dfrac{Q_3-Q_1}{Q_3+Q_1}$`

where

- $Q_1$ is the first quartile of the data
- $Q_3$ is the third quartile of the data