First introduced by Karl Pearson, an English mathematician and bio-statistician credited with establishing the discipline of mathematical statistics, a histogram is a graph figure which is used to display past data. It differs from the more well-recognized bar graph because a bar graph relates two variables, but a histogram relates only one (i.e., “earnings per month” in our example.
More specifically, histograms represent the distribution of numerical data, providing an estimate of the probability distribution of the continuous variable. Data within a histogram is displayed in “bins” and each bin has the same width. The example above uses $25 as its bin width and shows how many people earned between $800 and $825 per month, $825 and $850 per month, and so on. In other words, the “frequency” of each.
Histograms often provide new insights into the dynamics of process performance by indicating the number of times (frequency) each outcome occurred. Note that the mode of this frequency distribution is between $900 and $925, which occurs some 150 times.
To make a histogram, follow the following simple steps:\
- On the vertical axis, place frequencies. Label this axis “Frequency” covering the total span of gathered data points. In the example above, the span ranges from 0 to 200.
- On the horizontal axis, place the lower value of each interval measured. In the example above the first “bin” represents earnings between $800 and $825 per month, followed by a bin representing earnings between $825 and $850 per month, and so on.
- Draw a bar extending from the lower value of each interval to the lower value of the next interval on the horizontal axis, and reaching up to the associated frequency measurement.