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 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