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The Complete Idiot's Guide to Statistics, 2nd Edition by Robert Donnelly

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Measures of Relative Position

Another way of looking at dispersion of data is through measures of relative position, which describe the percentage of the data below a certain point. This technique includes quartile and interquartile measurements.

Quartiles

Quartiles divide the data set into four equal segments after it has been arranged in ascending order. Approximately 25 percent of the data points will fall below the first quartile, Q1. Approximately 50 percent of the data points will fall below the second quartile, Q2. And, you guessed it, 75 percent should fall below the third quartile, Q3. To demonstrate how to identify Q1, Q2, and Q3, let’s use the following data set.
def•i•ni•tion
Quartiles measure the relative position of the ...

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