Frequency table

A computer is normally used to draw histograms. Hand-drawn histograms are based on a frequency table that lists the histogram classes and their frequencies.

To avoid ambiguity in the histogram, the class boundaries should be chosen to ensure that no data values are on boundaries. For example,

Height of a histogram rectangle

To draw a histogram by hand with equal class widths, each class rectangle can be drawn with height equal to its class frequency. If class widths vary, we need to calculate the density for each class with the formula:

and use this for the rectangle heights.