Frequency histograms are the ultimate tool for visualizing process capability. The height of each bar on a histogram shows how often a given range of values occurs in our data.   Histograms can be made manually, but there are a number of software tools available to get the job done easily.

Consider each bar on a histogram a “bucket” that is assigned to a given range of values, with the height of each bar representing a number of data points that fall into each “bucket.”

The following histogram shows 50 tire pressure readings from an outgoing product audit at a motorcycle factory:

Histogram Example

Note that the x-axis is divided into “buckets” or cells, and the y-axis shows how many data points fall inside each bucket:

Tire Pressure Histogram Explained

Finally, we can add specification limits to graphically show how capable the process is:

Histogram with Specification Limits

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