Histograms

A histogram is a graph that displays the distribution of a quantitative variable.

In a histogram data is divided into classes, with individual values grouped into those classes. The number of individuals in each class is the height of the bar drawn on the histogram above the class limits.

For example:
If a group of students had the following weights in kg:
49, 52, 56, 72, 81, 89, 67, 87, 56, 67, 73, 87, 91, 56, 103, 79 and 82.
A histogram of these weights could look like the graph below.

You choose the number of classes for a histogram, too many or too few will effectively make the histogram meaningless.
Computer statistics packages like SPSS or SPLUS generally choose the best classes automatically.