QM - Six Sigma

7/30/2019 QM - Six Sigma

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Chapter 20 Six Sigma

Management


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Six Sigma (a registered trademark of the Motorola

Corp.) is the relentless and rigorous pursuit of the

reduction of variation in all critical processes to achievecontinuous and breakthrough improvements that impact

the bottom line of the organization and increase customer

satisfaction.

It is an organizational initiative designed to createmanufacturing, service and administrative processes that

produce approximately 3.4 defects per million.

Opportunities (DPMO).


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Relationship between the Voice of the Customer

and the Voice of the Process

Six Sigma management promotes the idea that the

distribution of output for a process (the Voice of the Process)

should take up no more than half of the tolerance allowed by

the specification limits (the Voice of the Customer)

This assumes that the measurement data is from a stable and

normal distribution of output whose mean can shift by as

much as 1.5 standard deviations over time.


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The following figure shows the Voice of the Customer asspoken in the language of a nominal value, m, and the

lower and upper specification limits, LSL and USL, for aquality characteristic.

This assumes

Distribution of process output is measurement data

The process is stable

The distribution is normal The process average is on nominal

The distance between nominal and either specification limit is 3times the process standard deviation

If these five conditions are met we all the process a 3-

sigma process.

A 3-sigma process will produce 2,700 defects per millionopportunities.


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LSL USL

3-sigma Process


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An Example of a 3-sigma Process

The number of days to complete a month accountingreport is stable and normally distributed with an average

of 7 days and a standard deviation of 1 day.

The lower specification limit is 4 days and the upper

specification limits is 10 days.


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Next we see a quality characteristic represented by

measurement data that:

Is stable Is normally distributed

Has a process output that can shift by as much as 1.5 standard

deviations on either side of nominal

Has a distance between nominal and either specification limit of 3

standard deviations of process output.

This is a 3-sigma process with a 1.5 sigma shift in the

mean.

This process will generate 66,811 defects per million

opportunities or 93.33189% of its output between thespecification limits.


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Accounting process with process average shifts of 1.5

standard deviations.


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Next we see a scenario where the voice of the process

takes up only half the distance between the specification

limits. The process mean remains the same, but theprocess standard deviation has been reduced to one half-

day


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Output for the monthly accounting process with the

standard deviation reduced to one-half day through

process improvement activities.


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Next we will see the voice of the customer and the voice

of the process for a quality characteristic that:

Is represented by measurement data

Is stable

Is normally distributed

Has a process average that can shift by as much as 1.5 standard

deviations on either side of nominal Has the distance between nominal and either specification limit is

6 standard deviations of the process output.

This is a process that will generate 3.4 defects per million

opportunities, or 99.99966% of its output within

specification limits in the near future.


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Six sigma accounting process with a 1.5 sigma shift in

the mean

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QM - Six Sigma