What is the purpose of establishing warning limits in statistical process control charts?

The purpose of establishing warning limits in statistical process control charts is to provide an early indication that the process may be drifting out of control, allowing for timely intervention before the process actually exceeds the control limits. Statistical process control (SPC) charts are graphical tools used to monitor a process over time and identify variations that may indicate a problem. These charts typically include a center line representing the process average, upper and lower control limits (UCL and LCL) representing the acceptable range of variation, and warning limits (also known as action limits) set between the center line and the control limits. The warning limits are typically set at ±2 standard deviations from the mean, while the control limits are set at ±3 standard deviations. When a data point falls outside the warning limits, it suggests that the process is deviating from its normal behavior and that there may be a special cause of variation at play. This triggers an investigation to identify the potential cause of the deviation and implement corrective actions to bring the process back into control. By acting on the warning signal, it is possible to prevent the process from exceeding the control limits, which would indicate a more serious out-of-control situation. For instance, if monitoring concrete compressive strength, a test result falling outside the warning limits but still within the control limits might prompt a review of the batching and mixing procedures to ensure consistency. This proactive approach helps to maintain process stability and prevent costly defects or non-conformances.