AAOM Handbook

INTERNAL

Appendix 1: Setting and Changing Control Limits Setting Initial Control Limits

Create and display the initial control chart, calculating temporary control limits using all of the data points. Ideally, you should have 25 to 100 points on the chart, but this is not an absolute requirement. There is no need to look at thousands of data points at this early stage. If the control chart is built on a boundary limited measure, and the calculated Control Limits lie outside of the boundary, set the value to the boundary value, eg if a measure has an upper value limited at 100, and the calculated Upper Control Limit is 115, manually set the upper Control Limit to 100. If there are points outside of the control limits on the chart, you should study each one, looking for the special cause. For any special cause you discover, do your best to remove it from the process – ensure that it will not happen again. Exclude the special causes from the calculation of the Control Limits and Mean and display the same control chart using newer Control Limits and Mean. Next, starting from the data at the beginning of the Control Chart, look for shifts, runs or changes in the range of the data that might indicate that the process changed. If a change appears to have occurred, and to have been sustained, calculate new control limits for the data up to the point where the change occurred and use temporary control limits (based on all of the data points occurring after the change has been established) for the rest of the chart. Repeat the above 2 steps until you reach the end of the data/chart. If there no shifts, runs or changes in the range of the data contained within the last section of the control limits on the chart extend these control limits as each new data point is added to the chart. At this point you can use the control chart for ongoing monitoring of the process. When you see an out-of control condition, it means that some special cause is present. This should be investigated and understood. Steps should be taken to ensure it does not happen again. If there is a shift, run or change in the range of variation evident in the data contained within the last points on the chart, it means that a special cause may be present. This should be investigated and understood, and the appropriate Control Action taken. Changing Control Limits If a change appears to have occurred in the process (i.e. there is a shift, run or change in the range of variation evident in the data) then a special cause is indicated and this should be investigated and understood immediately, and the appropriate Control Action taken. If the change in the process is only temporary then do not alter the Control Limits.