The two most common methods for calculating interquartile range are the exclusive and inclusive methods. · The exclusive method excludes the median when identifying Q1 and Q3, while the inclusive method includes the median as a value in the data set in identifying the quartiles. · For each of these methods, you’ll need different procedures for finding the median, Q1 and Q3 depending on whether your sample size is even- or odd-numbered. The exclusive method works best for even-numbered sample sizes, while the inclusive method is often used with odd-numbered sample sizes.

Now we can answer the question, "What is interquartile range?" We have shown that the IQR is simply the difference in the values of the first and third quartiles, Q1 and Q3. In order to find the IQR, simply subtract the smaller quartile from the larger one.

The interquartile range is the best measure of variability for skewed distributions or data sets with outliers. Because it’s based on values that come from the middle half of the distribution, it’s unlikely to be influenced by outliers.