The Interquartile Range

Q1 and Q3 = the lower and upper limit of the interquartile range.

The interquartile range = Q3 - Q1

Q1 = the 25th percentile. Q2 = the 75th percentile.

The 25th percentile is the value that has at most 25% of the observations below it and at most 75% of the observations above it.

The 75th percentile is the value that has at most 75% of the observations below it and at most 25% of the observations above it.

Therefore, the interquartile range (usually denoted as IQR) contains 50% of the observations.

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