A Unit and Group Runs Control Chart to Detect Shifts in the Process Mean

In this paper, we propose a control chart, called the ‘Unit and Group Runs’ (UGR) control chart useful to detect shifts in the process mean. The proposed procedure starts with the unit level inspection and switches to the group level inspection and back to the unit level inspection according to the specific rule. It is verified that, the UGR control chart gives a significant reduction in out-of-control ‘Average Time to Signal’ (ATS) as compared to the Shewhart’s X¯ chart, the synthetic chart (Wu and Spedding [12]), the ‘Group Runs’ (GR) chart (Gadre and Rattihalli [4]), the ‘Exponentially Weighted Moving Average’ (EWMa) chart (Roberts [10]) and the ‘Cumulative Sum’ (CUSUM) chart (Page [9]). In steady state also, the performance of UGR chart is remarkable as compared to the X¯ chart, the synthetic chart and the GR chart. Therefore, the UGR control chart is useful to enhance the effectiveness of statistical process control for variables.