The control chart is a graph used to study how a process changes over time. Data are plotted in time order. A control chart always has a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. These lines are determined from historical data.
The diagram below is an example of a control chart tracking the number of occurrence of defects (Y axis) over a period of time (X axis). Based on the historical data of the process, the average number of defects per month is 5.46 (Mean). The Upper Control Limit (UCL) is 9.09, The Lower Control Limit (LCL) is 1.83.
Any data point falling between the UCL and the LCL is considered as safe. The data points falling outside the LCL and the UCL are called ‘Outliers’. All outliers are candidates for Root Cause Analysis.
Whenever we see a cluster of seven data points happening next to each other, we call it as ‘Rule of Seven’. All ‘Rules of seven’ patterns calls for Root Cause Analysis.
The root cause analysis performed on the outliers and patters of rules of sevens can reveal two types of causes. Assignable causes and Random causes. All assignable causes are actionable, where as for random causes corrective actions are difficult.
Examples of assignable causes
- Poor quality of material used
- Poor workmanship
- Delayed delivery by the sub-contractor
Examples of random causes
- Earth quake
- Political instability