In control doesn’t mean acceptable results and vice versa

July 22, 2013

sigmaI had occasion to see something that merits comments. If you have a high Sigma value (meaning Six Sigma), that’s of course good but it’s not a signal to relax your quality control scheme. The reason is that Six Sigma and quality control are measuring two different things.

Six Sigma is measuring how many results are “acceptable” where acceptable is specified by the user as the total allowable error (TEa). Six Sigma doesn’t care (know) if the assay is in control.

“In control” is defined by the user as to whether the results satisfy the control rules chosen.

So you could have very tight TEa and an assay that is in control resulting in a low Sigma or, an assay with a very lenient TEa which has gone out of control but still has a very high Sigma.