What is a C-Chart?

The control chart (c-chart) for defects is also called the Attribute Chart (counting the number of defects). It is used to track the number of defects within units with constant sizes. The chart can track the number of defects per unit, whether there is a single defect type or multiple types. It assumes that the data are close to the distribution. The unit can be a specific section or an individual item, for example, the number of soldering defects on a printed-circuit board, or scratches on plated metal.

The c chart is used to determine the number of defects that are present in each defective sample or unit. P charts analyze the proportions or non-conforming items within a process.

On the c-chart, the y-axis plots the total number of defects, and the x-axis the number of units. The total number of defects is divided by the sample number to form the centerline (c) of the c-chart.

Selecting Control Chart

The Control Chart can be used to track the evolution of a process. The control chart has three lines: a central line representing the average, an upper and lower line representing the upper and lower control limits. The control limits are +-3s away from the centerline.

Control chart mapping is not complete without the selection of an appropriate chart.

An x-R chart is used to represent measurable quantities, such as weight and height. Attribute Control Charts are for attribute data. The data is the count of defective units or defective items. As an example, the number of tubes that failed in a shop. For attributes, unlike variable charts, there is only one chart.

Why and when do you use the C-Chart?

The c chart, one of the charts for quality control, is used to monitor the number of defects on a product with a constant size. The u chart is then used when the size of the product changes.

The c-chart can be used to check if a process is stable and predictable and to track the impact of process improvements before and after.It is used especially when the number of defects is low but there is a high chance for defects to occur in the subgroup.

The c chart is a control limit calculation based on Poisson distribution.

There are four types of control charts for attribute data. p graph plots the percentage of defective items and the number chart plots defectives. The u chart shows the average defect rate per unit, and the c chart shows the total number of defects.