5th Edition PMBOK Guide–Chapter 8: Quality Control


The quality of a deliverable is measured against the technical requirements obtained from the customers or sponsors of the project.    Process 8.3 Perform Quality Control is a process in the monitoring and controlling process group.    The monitoring part consists of sampling and inspecting the deliverables to see if the company can verify  the quality.   The controlling part comes if the company inspects the deliverables and see that they do not meet the technical requirements.

In that case, changes are recommended which can fed into the Perform Integrated Change Control process to see if they should be implemented or not.   Once the deliverables are verified, then can be validated by the customer or sponsor by seeing if they formally accept them as conforming to the original requirements that they set forth at the beginning of the project.

The following are three paired concepts that need to be understood with respect to quality control.

1.  Prevention and inspection

Prevention keeps errors from occurring during production.   Inspection tries to keep errors that have already occurred during production from ever reaching the customer.   Obviously PMI prefers as a matter of policy to have prevention emphasized over inspection, because it is ultimately cheaper to design in quality than “inspect” it in.

2.  Attribute sampling and variables sampling

Attribute sampling is to variables sampling as digital is to analog.   Attribute sampling just asks the question: does the sample conform or does it not conform?    Is is therefore like a digital value that could be assigned a 0 or a 1.  Variables sampling takes a measurement that is along a continuous (analog) scale, and thus the degree of conformity can be measured.

3.  Tolerances and control limits

The tolerance of a measurement is the specified range of permitted or acceptable results.   A control limit is a boundary set in order to keep all of the measurements within a specified tolerance.    The difference between a tolerance and control limit is like the difference on a highway between a guard rail and a lane marker.   The guard rail is like the tolerance limit:   if you go beyond that guard rail, you are not conforming to the road and this can be a very dangerous situation.   In order to keep you from going beyond the guard rail, there are lane markers painted on either side of a lane.  If you steer your car so that it always stays within the lane markers, then you will never have a problem having your car go off the road by going beyond the guard rail.

The concept of a control limit can be illustrated by the control limit chart tool, which is the subject of the next post.

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