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Part Four
Improvement
Defects per million
Zero defect
produced by the process, in terms of defects per million. The defects per million measure is
used within the Six Sigma approach to emphasize the drive towards a virtually zero defect
objective. Now the definition of Six Sigma has widened to well beyond this rather narrow
statistical perspective. General Electric (GE), who were probably the best known of the early
adopters of Six Sigma, defined it as, ‘A disciplined methodology of defining, measuring,
analysing, improving, and controlling the quality in every one of the company’s products,
processes, and transactions – with the ultimate goal of virtually eliminating all defects’.
So, now Six Sigma should be seen as a broad improvement concept rather than a simple
examination of process variation, even though this is still an important part of process
control, learning and improvement.
Measuring performance
The Six Sigma approach uses a number of related measures to assess the performance of
operations processes.
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A defect is a failure to meet customer-required performance (defining performance
measures from a customer’s perspective is an important part of the Six Sigma approach).
A defect unit or item is any unit of output that contains a defect (i.e. only units of
output with no defects are not defective, defective units will have one or more than one
defect).
A defect opportunity is the number of different ways a unit of output can fail to meet
customer requirements (simple products or services will have few defect opportunities,
but very complex products or services may have hundreds of different ways of being
defective).
Proportion defective is the percentage or fraction of units that have one or more defect.
Process yield is the percentage or fraction of total units produced by a process that are
defect-free (i.e. 1 − proportion defective).
Defect per unit (DPU) is the average number of defects on a unit of output (the number
of defects divided by the number of items produced).
Defects per opportunity is the proportion or percentage of defects divided by the total
number of defect opportunities (the number of defects divided by (the number items
produced × the number of opportunities per item)).
Defects per million opportunities (DPMO) is exactly what it says, the number of defects
which the process will produce if there were one million opportunities to do so.
The Sigma measurement is derived from the DPMO and is the number of standard
deviations of the process variability that will fit within the customer specification limits.
Worked example
An insurance process checks details of insurance claims and arranges for customers
to be paid. It samples 300 claims at random at the end of the process. They find that
51 claims had one or more defects and there were 74 defects in total. Four types of
error were observed, coding errors, policy conditions errors, liability errors and notification
errors.
Proportion defective =
Number of defects
Number of units processed
51
= = 0.17 (17% defective)
300
Yield = 1 − Proportion of defectives
= 1 − 0.17 = 0.83 or (83% yield)