When the number of observations is quite large, the principle according to which a distribution or an ordered data set is divided into two equal parts, may be extended to any number of the divisions. The three values which divide the distribution into four equal parts are called quartiles. These values are denoted by Q1, Q2 and Q3. Q1 is called first and lower quartile and Q3 is known as the third and upper quartile. The difference between the third and first quartile is called inter quartile range. Moreover, inter quartile range is a measure of dispersion of a data set.

105, 115, 119, 119, 123, 125, 127, 128, 128

To find the inter quartile range you take away the lower quartile (Q1) from the upper quartile (Q3) and the answer you get is the inter quartile range. Just in case you wanted to know, the semi-inter quartile range is the inter quartile range divided by 2, and the answer is the semi-inter quartile range (:

You subtract the 1st quartile from the 3rd quartile.