What is a Histogram?

It is a bar chart that represents a frequency distribution. The horizontal axis of the histogram represents the number of data points within a range (an interval), called a bin or class and the vertical axis represents the corresponding frequency. Normally, one would choose a range that best fits their data. The rule of thumb is to use 5-20 bins. If your bins are more than 20 your graph will be hard to read and if there are fewer than 5 your graph will have little, if any meaning.

Properties of a Histogram

1. Uses quantitative data
2. Has no gaps between bins (classes)
3. Bar width (bin size or class size) is constant
4. Vertical (Y) axis corresponds to frequency

How to Construct a Histogram

1. Create a frequency table
2. Setup intervals or bins with same width
3. Tally data in correct interval then calculate corresponding frequency
4. Use interval or bins along horizontal (X) axis and the frequency along vertical (Y) axis
5. Plot the data
6. Add title and legend

When to Use a Histogram

• When the data are numerical
• To summarize large data sets graphically
• When you want to see the shape of the data’s distribution
• When analyzing if a process meets customer’s requirements or not
• When determining whether a process change has occurred from one-time period to another
• When determining whether the outputs of two or more processes are different
• When you wish to illustrate distribution of data to others

A histogram is an essential process improvement tool as it enables a team to recognize and analyze data trends that are not seemingly obvious when looking at data tables. It is also an illustration of data distribution that includes the data’s spread and shape. Through analysis of the spread and shape of a distribution, the extent of variation in a process can be determined. This is essential as it can provide the platform to identify the cause of variation and the solutions needed to either control, reduce or eliminate it.

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