## Boxplot

A boxplot is a schematic presentation of summary statistics: a measure of location (sample median), a measure of dispersion (IQR between upper and lower sample quartiles), and the largest and smallest data. It also indicates the symmetry or skewness of the distribution and the presence of possible outliers.
A box in the middle of boxplot
stretches from the lower quartile `Q1` (the 25-percentile)
to the upper quartile `Q3` (the 75-percentile).
The median `Q2` is shown as a line across the box.
Therefore, 1/4 of the observations lies between this
line and the right of the box and 1/4
of the observation between this line and the left of the box.
Straight lines (either horizontal or vertical)
are called whiskers, stretching out from
the ends of the box to the largest and smallest value, `Q0` and `Q4`.
In the computer-generated boxplot potential outliers will be
indicated separately from whiskers.
The plot will be
displayed.

The data of one variable are often grouped by another categorical variable. Such a categorical variable is often referred as ``factor,'' and its values are called ``levels.'' The presentation of side-by-side boxplots is quite useful for comparing the data from different groups (i.e., different levels).