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of the lower and upper half of your data. The median is the "midpoint," or the number that is halfway into a set. In this case, you aren't looking for the midpoint of the entire set, but rather the relative midpoints of the upper and lower subsets. If you have an odd number of data, do not include the middle number – in Set B, for instance, you would not figure in one of the 10s.  Even example (Set A):  Median of lower half = 7 (Q1) Median of upper half = 12 (Q3)   Odd example (Set B):  Median of lower half = 8 (Q1) Median of upper half = 18 (Q3) Now you know how many numbers lie between the 25th percentile and the 75th percentile. You can use this to understand how widely-spread the data is. For instance, if a test is scored out of 100, and the IQR of the scores is 5, you can assume that most of the people taking it had a similar grasp of the material because the high-low range is not very large. If the IQR of the test scores is 30, however, you might start to wonder why some people scored so high and others scored so low.  Even example (Set A): 12 - 7 = 5 Odd example (Set B): 18 - 8 = 10

Summary:
Find the median Subtract Q3 - Q1 to determine the IQR.