Stats tip: A run of 5? Or 6? The debate continues.

Matt SavageI received a lot of e-mails  in response to my last blog entry and a few of you have posted comments on the blog. It is fantastic to see such a lively debate! And while I like to win a debate as much as the next guy, I am more concerned with utility.  After all, if your control charts don’t give you information that helps you control your processes, what’s the point?

We must remember the purpose of a control chart, which is to provide guidance as to when to investigate and when not to investigate. Essentially, a control chart should create an effective balance between reacting too quickly and not reacting quickly enough. While it is evident that we aren’t all using the same operational definition of a run, the nice thing about SQCpack and CHARTrunner is that they allow you define your own run rules.

Note the two distinct runs of points between samples 8 and 18 on the following chart of fictitious data.

Both CHARTrunner and SQCpack count runs in a manner consistent with many SPC texts. Thus, both software packages would count two runs of five samples each in this example.

If you see a run of five (5) consecutive increasing points (samples) and then a run of five (5) consecutive decreasing points and you want to be alerted when you have six (6) consecutive points increasing or decreasing, then there’s no need to adjust the software. The default settings in both SQCpack and CHARTrunner will match your expectations. This chart would be considered in-control by the default out-of-control tests.

On the other hand, if you count a run of six (6) consecutive increasing points and then a run of six (6) consecutive decreasing points and you want to be alerted when you have six (6) or more consecutive increasing or decreasing points, simply define the out-of-control test that you are using to test for “5 ascending” and “5 descending.” You will get the results you want, meaning samples thirteen and eighteen will  be flagged as out-of-control conditions .

Now that you know how CHARTrunner and SQCpack count runs, you can ensure the software properly alerts you. And isn’t that the whole point? I’m interested in your thoughts. Keep the questions and comments coming.