Find: 1. three quartiles and 2. interquartile range (IQR).
1, 3, 9, 12, 12, 14, 18

The position of Q1 is the 2th number. Q1 is 3.
The position of Q2 is the 4th number. Q1 is 12.
The position of Q3 is the 6th number. Q1 is 14.
IQR is 11.
The first quartile, second quartile and the interquartile range are used to measure the variation of data. Median is a measure of central tendency. Median is the number at the center of the dataset.
Q1 = 1/4 x (n + 1)
1/4 x 8 = 2nd number = 3
Q2 = 1/2 x (n + 1)
1/2 x 8 = 4th number = 12
Q3 = 3/4 x (n + 1)
3/4 x 8 = 6th number = 14
IQR = Q3 - Q1
14 - 3 = 11
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