Understanding Histograms

A histogram differs from a bar chart in that it is the area of the bar that denotes the value, not the height as in bar charts. The histogram is one of the seven basic tools of quality control, which also include the Pareto chart, check sheet, control chart, cause-and-effect diagram, flowchart, and scatter diagram.

Histograms are best used when there is large amount of data presented in a table. Histograms are like a snapshot as opposed reflecting a process' performance over time like run charts and control charts do. A histogram makes it easy to see where the majority of values fall in a measurement scale, and how much variation there is. You will want to construct a histogram when you want to do the following:

• Summarize large data sets graphically. A data set presented in a table isn’t easy to use. You can make it much easier to understand by summarizing it on a tally sheet and organizing it into a Histogram.
• Compare process results with specification limits. If you add the process specification limits to your Histogram, you can determine quickly whether the current process was able to produce "good" products. Specification limits may take the form of length, weight, density, quantity of materials to be delivered, or whatever is important for the product of a given process.
• Communicate information graphically. The team members can easily see the values, which occur most frequently. When you use a Histogram to summarize large data sets, or to compare measurements to specification limits, you are employing a powerful tool for communicating information.
• Use a tool to assist in decision-making. Certain shapes, sizes, and the spread of data have meanings that can help you in investigating problems and making decisions. Bear in mind that if the data you have in hand aren’t recent, or you don’t know how the data were collected, it’s a waste of time trying to chart them. Measurements cannot be used for making decisions or predictions when they were produced by a process that is different from the current one, or were collected under unknown conditions.

A Histogram is made up of five parts:

Title: The title briefly describes the information that is contained in the Histogram.

Horizontal or X-Axis: The horizontal or X-axis shows you the scale of values into which the measurements fit. These measurements are generally grouped into intervals to help you summarize large data sets. Individual data points are not displayed.

Bars: The bars have two important characteristics—height and width. The height represents the number of times the values within an interval occurred. The width represents the length of the interval covered by the bar. It is the same for all bars.

Vertical or Y-Axis: The vertical or Y-axis is the scale that shows you the number of times the values within an interval occurred. The number of times is also referred to as "frequency."

Legend: The legend provides additional information that documents where the data came from and how the measurements were gathered.