Histograms The histogram is a tool for presenting the distribution of a numerical variable in graphical form. For example, suppose the following data is the number of hours worked in a week by a group of nurses:

Histograms The vertical axis is frequency. So, for example, there are two nurses who worked from 25 to less than 30 hours that week. The data values are grouped in intervals of width five hours. The first interval includes the values from 25 to less than 30 hours. The second interval includes values from 30 to less than 35 and so on. The intervals are shown on the horizontal axis These data are displayed in the following histogram:

Histograms The choice of interval width will affect the appearance of the histogram. To the right is the same data presented in a histogram of interval width 10. And here it is again, to the right, presented in a histogram of interval width 2.