The interquartile range (IQR) is a measure of the middle fifty percent of the observations in a data set – often also referred to as the “arm’s length-range”.
(See TPG 2017, para. 3.57 – 3.62 for guidance on application)
Example – finding the IQR of a set of observations:
Step 1: Put the observations in order and finde the median.
1%, 2%, 5%, 6%, 7%, 9%, 12%, 15%, 18%, 19%, 27%.
Step 2: Place parentheses around the numbers above and below the median. Q1 (the lower quartile) is the median in the lower half of the observations and Q3 (the higher quartile) is the median for the upper half of the observations.
(1%, 2%, 5%, 6%, 7%), 9%, (12%, 15%, 18%, 19%, 27%).
In this example, observations higher than or equal to 5% and lower than or equal to 18% would be said to be “within the interquartile range”.