In the previous article in this series (June, page 21) we associated required performance with the complexity of control required. Complexity is determined by the application of rules. Every rule is triggered by a particular pattern of results and thereby detects error we need to control – either random or systematic, but not both. Here, we compare single and multiple rule approaches.
Rules are often described as warning, rejection or investigate further. Rejection rules are simple to understand. If they are breached, the process must be stopped. In the past, compliance to the 12S rule was satisfactory. Further investigation was not required. Nowadays, further investigation can be instigated before rules are breached. This is particularly true for trend and systematic shift rules that use multiple sequential runs and allows the scientist to identify whether there is an issue prior to the actual alarm.
The simplest and most familiar of