Interquartile Range
A measure of spread calculated as IQR = Q₃ - Q₁, where Q₃ is the upper quartile and Q₁ is the lower quartile. It represents the range of the central 50% of the data.
Real World
When the NHS publishes A&E waiting-time data, the IQR shows how long the middle 50% of patients waited, filtering out the very quickest and the unusually long waits caused by major incidents.
Exam Focus
Show your quartile positions clearly — write Q₁ is at position (n+1)/4 — and use interpolation for grouped data to secure method marks.
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