Control Charts

A Control Chart is a Time Plot with the added features of upper and lower control limits (a.k.a.; "UCL" and "LCL"). Data points which fall between the UCL and the LCL are typical and expected. Data points which fall above the UCL or below the LCL, or that fall into a particular pattern, indicate a special cause of variation requiring further investigation and possibly correction (or mitigation, if that is all that is possible). You will notice in the example below that at 11:10 the number of chips exceeded the UCL. Regrettably, the corrective action taken to fix the problem was excessive, sending the process out of control in the other direction.