The simplest measure of spread is the range. The range is the difference between the maximum and minimum values.

The median of a sample cuts the data set in half. If we cut the halves in half, such that the data is divided into quarters, we can find the quartiles of a data set. The first quartile calculates the value one quarter of the way up the list. The second quartile calculates the value two quarters up the list (the median value).The third quartile calculates the value three quarters of the way up the list and so on. Quartiles are useful in determining whether or not a distribution is symmetric about the median.

The interquartile range is the difference between the first and third quartiles. This is often favored over the range as it is not impacted by outlying low and high values.