Interquartile Range
Also called: IQR, quartiles, box plot
The interquartile range is the distance between the 25th and 75th percentiles — the width of the middle half of the data. It's a measure of spread that ignores the tails entirely, making it robust where standard deviation is not. The common outlier rule of thumb, flagging anything beyond 1.5 IQRs from a quartile, is what draws the whiskers on a box plot.
Formula
IQR = 75th percentile − 25th percentile
Where this is taught
Definitions are the trailer. These free levels turn Interquartile Range into something you play — one bite-size lesson, with worked examples, a quiz and XP.