IQR is Q3 minus Q1: the spread of the middle half of the data. It ignores the tails, so one extreme value barely moves it. Box plots show that box, in box plots.
Middle fifty, not the whole range
Range is max minus min and is sensitive to a stray point. An outlier sits outside the typical fence, in outliers. Mean and median are location, not spread, in mean, median, and mode.
Standard deviation is another spread measure, in standard deviation.
The bottom line
IQR = Q3 − Q1. Use it when the question wants the middle, not the extremes.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.