Control charts

Control charts are the basic tool of statistical process control.

Absolute control over the whole production process is unachievable. Hence, a careful selection of the most relevant variables that characterize the process is required.

Depending on the selected variables, different charts are used:

• Continuous variables - x and R charts: weight, length, process time...
• Discrete variables - p and c charts: number of defective pieces, number of defects per product...

Control charts require sampling from the production process. Samples are grouped and mean values from each group presented in a chart, together with three lines:

• Upper control limit (UCL)
• Central line
• Lower control limit (LCL)

Sampled values above the UCL or below the LCL are indicative of special causes of variation.