Control Charts Rules

[Filipa]

Good night.
I would like to know your opinion about the western and nelson rules for control charts.
How to select the rules to apply to control charts? Based on what and why?
Thank you

 

[Miner]

First of all, do not attempt to apply all of these rules to any given process or you will end up with more false alarms, Second, rules 4 & 8 (from Wikipedia listing) are intended as diagnostic tests to ensure rational subgroups, so these are only to be used when setting up a control chart then dropped when you are sure of your subgrouping. Third, rules 5, 6 & 7 are only applied when your process needs tighter control than rule 1 provides; if you have a high Cpk, these are unnecessary. Finally, rules 2 or 3 should only be used if your process is subject to runs or trends.