The control chart is a statistical tool that represents the performance of a process over a given time interval. It shows the desired performance/measurement (depicted by the central line in the plot), the upper and lower controls of the process (shown by lines above and below the central line). Any data points above the upper control line represent a special cause of variation and are due to an employee's error or unusual circumstances. Any data point below the lower control line represents a common cause of variation and is attributed to process design errors.
The control charts are used as a visual tool for the operators and managers to monitor the performance over time and take corrective steps when the process is not in control. The control chart represents the process performance over time and depending on the type of control chart used, may represent: process stability (p-chart); number of defective parts (np plot), etc.