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The lower quartile, median and upper quartile are usually denoted by Q_1, Q_2 and Q_3. To find them you must first arrange the data in order of size.
If n is odd, then Q_2 is the \frac{1}{2}(n+1)th value. If n is even the median is halfway between the \frac{1}{2}nth value and the next.
To find Q_1 find the median of the lower half of the data. To find Q_3, find the median of the upper half of the data.
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Glossary

median

the middle value of a set of data that has been arranged in order of magnitude; half the data values are above it and half are below.

quartile

In an ordered set of data 25% are below the first (Q1), 50% are below the second (Q2), 75% are below the third (Q3).

Full Glossary List