Histogram

Purpose: To graphically depict the frequency distribution of data items using a vertical bar chart (columns) format.

Method:

• Collect data on a particular variable.
• Generate a frequency table listing all the data points. •Count the number of data points.
• Determine the range of values for the data (maximum value minus the minimum value).
• Determine the number of bars to depict in the chart. One common method is to use the square root of the number of data points; e.g., 100 data points = 10 bars; 225 data points = 15 bars, etc.
• Calculate the intervals represented by each of the bars. The simplest method is to divide the range of values by the number of bars (from previous step).
• Determine the frequency of data points within each interval of values.
• Create a vertical bar chart with a vertical bar (column) for each of the variable values or range of values you measured on the horizontal axis. The height of the bar will equal the frequency (on the vertical axis) for each of the values/ranges.

In the example below, the sample size is 220 data points (N = 220).

The square root of 220 is between 14 and 15, so either will work for the number of bars in the chart (14 bars are used in the example).

The range of values is 350 hrs (1350 hrs minus 1000 hrs).  Dividing the range (350) by the number of bars (14) results in intervals of 25 hrs.

Example