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There are observations, so the median is the middle number of the arranged set of data. Splitting the observations either side of the median gives two groups of observations. The median of the lower half of data is the lower or first quartile. The median of the upper half of data is the upper or third quartile.

The median of the lower half of data is the lower or first quartile

The median of the upper half of data is the upper or third quartile

Arrange the terms in ascending order.

The median is the middle term in the arranged data set.

The lower half of data is the set below the median.

The median is the middle term in the arranged data set.

The upper half of data is the set above the median.

The median is the middle term in the arranged data set.

The interquartile range is the difference between the first quartile and the third quartile . In this case, the difference between the first quartile and the third quartile is .

Multiply by .

Subtract from .

Find the Interquartile Range (H-Spread) 2 , 5 , 9 , 11 , 18 , 30 , 42 , 48 , 71 , 73 , 81