The difference between the first (lower) and third (upper) quartiles of a variable (Q3 – Q1). This is the preferred measure of variation where there is a skewed distribution, in order to disregard outliers.
To measure the lower quartile, where n is the number of observations you need to calculate 1/4 of n+1. E.g. if n=18 then the IQR is 4.75, which means it is three quarters of the way between the 4th and 5th ordered values.
To measure the upper quartile, you need to calculate 1/4 of n+1 *3. E.g. if n=18, 1/4 of 57 = 14.25 so the third quartile lies one quarter of the way between the 14th and 15th ordered values.