Before we move on, let’s define some terms that are commonly used in exploratory data analysis.

Percentiles are special cases of quantiles that are commonly used. The percentiles are the quantiles you obtain when setting the \(p\) at \(0.01, 0.02, ..., 0.99\). We call, for example, the case of \(p=0.25\) the 25th percentile, which gives us a number for which 25% of the data is below. The most famous percentile is the 50th, also known as the median.

For the normal distribution the median and average are the same, but this is generally not the case.

Another special case that receives a name are the quartiles, which are obtained when setting \(p=0.25,0.50\), and \(0.75\).