Click here to purchase the entire book in PDF format. Percentile 1. Put your measurements in order from smallest to largest 2. the $p$th percentile is the value of $x$ that exceeds $p\%$ of the measurements and is less than $(100-p)\%$ Figure 4.1: A graphical representation of the 70th percentile in a standard distribution of values. The 25th percentile is also known as the lower quartile, first quartile or $Q_{1}$. The 50th percentile is also known as the mean The median is the second quartile or $Q_{2}$ The 75th percentile is also known as the upper quartile, third quartile or $Q_{3}$ The Interquartile range is $Q_{3} - Q_{1}$