Tool: Control Chart 3
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Advantages of Statistical Control
1. Can predict its behavior.
2. Process has an identity.
3. Operates with less variability.
4. A process having special causes is unstable.
5. Tells workers when adjustments should not be made.
6. Provides direction for reducing variation.
7. Plotting of data allows identifying trends over time.
8. Identifies process conditions that can result in an
source: Juran and Gryna, Quality Planning and Analysis, p. 380-381.
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source: Donald Wheeler, Understanding Statistical Process Control
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Wheeler, Understanding Statistical Process Control
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“Take this example: In finance we set a budget. The actual expenditure, month by
month, varies - we bought enough stationery for three months, and that’s going to be
a miniblip in the figures. Now, the statistician goes a step further and says, ‘How do
you know whether it’s a miniblip or there’s a real change here?’ The statistician says,
‘I’ll draw you a pair of lines here. These lines are such that 95% of the time, you’re
going to get variation between them.’
Now suppose something happens that’s clearly outside the lines. The odds are
something’s amok. Ordinarily this is the result of something local, because the
system is such that it operates in control. So supervision converges on the scene to
restore the status quo.
Notice the distinction between what’s chronic [common cause] and what’s sporadic
[special cause]. Sporadic events we handle by the control mechanism. Ordinarily
sporadic problems are delegable because the origin and remedy are local. Changing
something chronic requires creativity, because the purpose is to get rid of the status
quo - to get rid of waste. Dealing with chronic requires structured change, which has
to originate pretty much at the top.”
A Conversation with Joseph Juran
Source: A Conversation with Joseph Juran, Thomas Stewart, Fortune, January 11, 1999, p. 168-170.