Quartiles - GCSE Maths Definition

The quartiles, of a data set, and the values that divide the data into four equal parts. The lower quartile (sometimes called Q1) is the data value one-quarter, or 25%, from the start, once the data has been ordered. The upper quartile (Q3) is the data value three-quarters, or 75% from the start. The 'second quartile' (Q2) is the data value one-half, or 50%, from the start - this is usually not referred to as a quartile as it has its own name - the median.

Finding quartiles, and so splitting the data into four equal parts, allows us to analyse and interpret the lowest group of values in a data set, and the highest, whilst 50% of the data lies between the two quartiles. This could be important where a business wants to aim its products at the 'middle 50%' of the population (for example, a clothes shop would stock more of the sizes that sell the most).