Type III Error

Control Charts that are Not Control Charts

Frequently "Control Charts" display Individual Values. The primary response to "Out of Control" with these Charts is generally not to review the Process for inadvertant changes. If so, then it would be a Control Chart, although it would be an insensitive one, since it is using Individual Values instead of a more precise Sample Mean, to detect changes in the Process Mean.

Sometimes the Response to "Out of Control" is primarily is to make adjustments. If so, the Chart is not a Control Chart but a "Feedback Control Chart." It is a sort of manual Automatic Process Control, better done with PID Condrol methods and others.

Commonly, when the Chart is monitoring a Product Parameter, especially a final Customer Specified Product Parameter, the primary response is "Containment." That is, the specific Lot or Lots or Segment of Product "associated with or "near" the "Out of Control" event is pulled for special handling and inspection. Only after it has been checked and found okay or else purged of potential questionable material is this material allowed to go to the Customer, or else it is "scrapped." The use of the Chart like this is not as a Control Chart. This is Acceptance Sampling put in a format of a Control Chart. The sampling plan is one unit from some Production Segment (like a Lot). This is a pretty lousy Acceptance Sampling Plan, especially if it is one size fits all and not set up this way for good reasons other than administrative simplicity.

Labels: Control Chart, Individual Values

posted by X-Bar @ 8:00 AM