Relative frequency

When all histogram classes are of equal width, histograms are often drawn with a vertical axis giving the frequencies (counts) for each class. Alternatively, the axis can be labeled to show the relative frequencies (proportions) in the classes.

Ages of patients admitted to cardiac unit

Area equals relative frequency

In a histogram, the area contributed by any value in the data set is the same. The proportion of the total histogram area for each value is:

where n is the number of values in the data set. Therefore,

The proportion of values (relative frequency) in any classes is the proportion of the total area above these classes.

Wood chip length