Histograms

A hand-drawn smooth curve on a stacked dot plot can describe the density of values well but is a subjective method — different people would draw slightly different curves to smooth out the irregularities in the stack heights. A histogram is an objective graphical display of a data set with the same objective.

In a simple histogram, the axis is split into sub-intervals of equal width called classes. A rectangle is drawn above each class with height equal to the number of values in the class — the frequency of the class.

Ages of patients admitted to cardiac unit

The stacked plot below shows the distribution of ages of patients admitted to a hospital's cardiac emergency unit during a four-month period.

Drag the slider to change the stacked dot plot into a simple histogram. Note that the height of each rectangle equals the number of crosses.


The diagram below generalises by allowing classes that are wider than the dot plot stacks.

Click on any histogram rectangle to verify that the rectangle height equals the number of values in that class.

Use the two buttons on the left to adjust the class width and observe that the rectangle heights are again equal to the frequency of values in the class.

Finally, hide the crosses by clicking the checkbox. Histograms generally do not display the individual values in a data set.