Quality Control Chart

A quality control chart is a graphic that depicts whether sampled products or processes are meeting their intended specifications and, if not, the degree by which they vary from those specifications. When each chart analyzes a specific attribute of the product it is called a univariate chart. When a chart measures variances in several product attributes, it is called a multivariate chart. Randomly selected products are tested for the given attribute or attributes the chart is tracking. A common form of quality control chart is the X-Bar Chart, where the y axis on the chart tracks the degree to which the variance of the tested attribute is acceptable. The x axis tracks the samples tested. Analyzing the pattern of variance depicted by a quality control chart can help determine if defects are occurring randomly or systematically.

Different types of quality control charts, such as X-bar charts, S charts and Np charts are used depending on the type of data that needs to be analyzed. A quality control chart can also be univariate or multivariate, meaning that it can show whether a product or process deviates from one or from more than one desired result.

For example, Bob wants to know if his widget press is creating widgets that are up to standard. He decides to test the density of a random sampling of widgets to see if the press air injection system is working properly and mixing enough air into the widget batter. An appropriately airy batch of widget batter will cause the finished widget to float in water. Bob creates an x-bar chart to track the degree to which each randomly selected widget is buoyant.