A modified approach to the computation of control limits forX̄andRcharts is introduced. This procedure consists of replacingX̄with the trimmed mean of the subgroup averages, andR̄with the trimmed mean of the subgroup ranges. Standard tables of control limits may continue to be used, requiring only adjustment by a constant multiplier, which is tabulated. The proposed control chart limits are shown to be less influenced by extreme observations than their classical counterparts, and to lead to tighter limits in the presence of out-of-control observations. These concepts are illustrated by several examples.