GMS 401 Chapter 10: Statistical Quality Control
Statistical Quality Control: use of statistical techniques and sampling in monitoring and testing quality
of goods and services
The part of statistical quality control that relies primarily on inspection/ test of previously
produced items is referred to as acceptance sampling
Statistical quality control is important because it provide an economical way to evaluate the
quality of products and meet the expectations of the customers
Inspection: appraisal of a good or service against a standard.
Effective statistical process control requires the following planning steps:
o Define the quality characteristics important to customers, and how each is measured
o For each characteristic
Determine a quality control point
Plan how inspection is done, what to expect, and whether centralized or on site.
Plan the corrective action
The amount of inspection can range from no inspection whatsoever to inspection of each item.
Items that have large cost associated with passing defective products require inspection
The cost of inspection and resulting interruptions of a process typically outweigh the benefits of
10 percent inspection
If inspection increases the inspection cost increases, if the cost decreases the cost of passing
The goal is to minimize the sum of those two costs
As a rule of thumb, operations with a higher proportion of human involvement necessitate more
inspection than mechanical operations, which tend to be more reliable.
A stable process will require only infrequent checks, whereas an unstable one, or one that has
recently given trouble will require more frequent checks.
Some situations require that inspections be performed on site.
Statistical Process Control
Statistical Process Control (SPC): statistical evaluation of the product in the production process
To do SPC the operator take periodic sample from the process and compares them with
predetermined limits. If the sample results fall outside the limits, the operator stops the process
and takes corrective action.
The variations are created by combined influences of countless minor factors, each one so
unimportant that even if it could be identified and eliminated, the decrease in process variably
would be neglected.
Random Variation: Natural variation in the output of a process, created by countless minor factors
Assignable Variation: non-random variability in process output; a variation whole cause can be
The main task in SPC is to distinguish assignable from random variation. Taking a sample of two
or more observations and calculating a sample statistic such as sample mean makes the task