A histogram is a graphic summary of variation in a specific set of data. The data which are presented in histograms are variables data, i.e. time, length, height, weight.
Steps in constructing the histogram:
The following four steps are recommended when constructing a histogram.
Step 1:Plan and collect the data. The data has been collected.
Step 2:Calculate the range of the data. The range is equal to the difference between the highest and the smallest number in the data set. In the case example the range is equal to (20–7) days=13 days.
Step 3:Determine intervals and boundaries. The purpose of this step is to divide the range into a number of equal broad intervals in order to be able to calculate the frequencies in each interval. The number of intervals depends on the number of data but both too few intervals and too many intervals should be avoided. A number of intervals between eight and 12 is normally a good rule of thumb.
Step 4:Determine the frequencies and prepare the histogram. Now the data in each can be constructed. Today when data is usually stored in a computer this step is unnecessary. For most software packages steps 2 to 4 are done interactively with a computer program.
Conclusion of the histogram:
The following are easily concluded from the histogram:
1. The standard of 13 days was met only in approximately two out of three cases.
2. There seem to be two different distributions mixed in the same histogram. Perhaps the left distribution is the result of simple suggestions and the right distribution is the result of more complex suggestions.
Drawback of the histogram:
- One weakness of the histogram is that you do not see a picture of the variation in time.
- For example the variation shown in a histogram may be the result of a combination of two or more different distributions.