What Is a Quality Control Chart and Why Does It Matter?

What Is a Quality Control Chart?

A quality control chart is a graphic that depicts whether sampled products or processes are meeting their intended specifications. If not, the chart will show the degree by which they vary from specifications. A quality control chart that analyzes a specific attribute of a product is called a univariate chart, while a chart measuring variances in several product attributes is called a multivariate chart. Randomly selected products are tested for the given attribute(s) the chart is tracking.

Understanding Quality Control Charts

Quality control (QC) is a set of processes through which a business ensures that product quality is maintained or improved. Quality control requires the business to create an environment in which both management and employees strive for perfection. This is done by training personnel, creating benchmarks for product quality, and testing products to check for statistically significant variations.

A major aspect of quality control is the establishment of well-defined controls. These controls help standardize both production and reactions to quality issues. Limiting room for error by specifying which production activities are to be completed by which personnel reduces the chance that employees will be involved in tasks for which they do not have adequate training.

Quality control charts are a type of control often used by engineers to assess the performance of a firm's processes or finished products. If issues are detected, they can easily be compared to their location on the chart for debugging or error control. In other words, it provides a heuristic blueprint for maintaining quality control.

A common form of the quality control chart is the x-bar (denoted as x̅) 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.

The R (range) chart is a quality control chart used to monitor the variation of a process based on small samples taken at specific times.

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.

Example of a Quality Control Chart

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.