A number does not have a quartile, a set of data does. The lower quartile of a set of data set is a value, in the data set, such that a quarter of the date set are smaller and three quarters are larger. The upper quartile is defined similarly. The middle quartile, better known as the median, divides the data set in two.
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The interquartile range is the difference between the Lower quartile and the upper quartile. Obviously you need to be able to find these values. Haylock (2006) explains how to do this for difficult size groups in mathematics explained for primary teachers. He explains the position of the lower quartile is a quarter of (n+1) and that of the upper quartile is three-quarters of (n+1). So for a group of 7 numbers, you find a quarter of 8, which is 2. Therefore the number in second place is the lower quartile. Three quarters of 8 is 6 and so the number in 6th position is the upper quartile. Now take the lower quartile from the upper quartile.
the IQR is the third quartile minus the first quartile.
Find the difference between the values for quartile 3 and quartile 1.
First Quartile = 43 Third Qaurtile = 61
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.