A control chart is a statistical tool used in quality control to monitor and control processes. It helps to determine how much variability in a process is due to random variation and how much is due to unique events.

Random variation, also known as common cause variation, is inherent in any process and is predictable1. It represents the natural fluctuation in a process over time due to many minor factors1.

On the other hand, unique events, also known as special cause variation, are unexpected and arise due to unusual circumstances23. They are not an inherent part of a process and are not predictable3.

A control chart helps distinguish between these two types of variation. If a data point falls within the control limits on the chart, it is considered to be due to random variation. If a datapoint falls outside the control limits, it indicates the presence of special cause variation1.