## About This Tool

The online Quartile Deviation Calculator is used to calculate the quartile deviation of a set of numbers.

## Quartile Deviation

In descriptive statistics, the quartile deviation (QD) is half the difference between the third (upper) and first (lower) quartiles.

## First Quartile And Third Quartile

The first quartile, also called lower quartile, is equal to the data at the 25th percentile of the data. The third quartile, also called upper quartile, is equal to the data at the 75th percentile of the data.

There are several different methods for calculating quartiles. This calculator uses a method described by Moore and McCabe to find quartile values. The same methord also used by The TI-83 to calculate quartile values. With this method, the first quartile is the median of the numbers below the median, the third quartile is the median of the numbers above the median.

## Formula

The following is the quartile deviation calculation formula:

QD = (Q3 - Q1)/2

where:

QD = quartile deviation

Q3 = third quartile

Q1 = first quartile