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The inter-quartile range.
Find the difference between the values for quartile 3 and quartile 1.
A quartile divides a grouping into four. The first quartile will have the first 25% of the group, the second quartile will have the second 25% of the group, the third quartile will have the third 25% of the group and the last quartile will have the last 25% of the group. For example if a classroom had 20 students who had all taken a test, you could line them up, the top 5 marks would be in the first quartile, the next five would be in the second quartile, the next 5 would be in the third quartile, and the 5 students with the lowest marks would be in the last quartile. Similarly, a percentile divides a grouping, except the group is divided into 100. Each 1% represent 1 percentile.
The distance between 67.8 and 70.8 on a box plot is known as the interquartile range (IQR). It is calculated as the difference between the third quartile (Q3) and the first quartile (Q1), which represent the limits of the box in the box plot.
lower quartile = 1/4(n+1) upper quartile = 3/4(n+1) where n is the number of the values. Obviously the values have to be ordered from the lower to the higher: the number you'll get is the position in this order. Let's say you get 4 for your lower quartile, it means that the 4th value is your lower quartile.