AAOM Handbook

INTERNAL

AI.02 Process Stable? Context

A Key Performance Indicator or Control Measure is produced for the purpose of helping us make the decisions necessary to control or improve a process. The first question to be asked when assessing process performance is whether the process is stable. This is not whether there is variation in the process (there always will be normal variation), or whether there is too much normal variation, but is whether there is an abnormal variation that indicates either an uncommon event or that the process has changed. The event or change may be un-intended, as when there is wear, drift or failure of a component of machinery used in the process, or where an operator has altered the setting of a parameter away from the specification. The change may also be intentional, as when a higher performance component or new process setting is being trialled. A stable process is one that is performing consistently, that is, nothing unusual has happened and the process is not changing. A control chart is used to determine if a process is stable. If the control chart indicates that the process is not stable the situation should be investigated immediately. If the control chart indicates that the process is stable, then the capability of the process should be reviewed. Purpose To determine if the process is stable. Quantity One decision on each KPI - is the process stable or not? Quality A test for stability should be conducted at the time each new measurement point is added to the control chart. The tests to be applied for determining stability are;  No data point outside of the control limits  No 7 consecutive points above or below the mean  No 7 consecutive points ascending or descending  No recurring tendency or pattern in the data Resources This task is completed by the person responsible for the process performance at the time a new measurement point is added to the control chart. Time Process stability should be reviewed each time a new data point is added to a control chart.