DefinitionControl - Definition of Control in SPC (Statistical Process Control)

jelly1921

Quite Involved in Discussions
SPC = statistical process control

When we say a process in a state of statistical control or in control, it means the process is stable, predictable and consistent.

Here the word of "control" is obviously not the meaning of "exercise restraint" or "direction over"; or "dominate"; or "command". Then what is it?

Stijloor

Staff member
Super Moderator
Re: Control in SPC

SPC = statistical process control

When we say a process in a state of statistical control or in control, it means the process is stable, predictable and consistent.

Here the word of "control" is obviously not the meaning of "exercise restraint" or "direction over"; or "dominate"; or "command". Then what is it?
In the SPC context, think about "Manage."

Stijloor.

Jim Wynne

Staff member
SPC = statistical process control

When we say a process in a state of statistical control or in control, it means the process is stable, predictable and consistent.

Here the word of "control" is obviously not the meaning of "exercise restraint" or "direction over"; or "dominate"; or "command". Then what is it?
Another definition of "control" is to hold or restrain; a process that's in control has been held or restrained within defined limits. Processes don't become stable all on their own--they must be controlled.

Stijloor

Staff member
Super Moderator
Another definition of "control" is to hold or restrain; a process that's in control has been held or restrained within defined limits. Processes don't become stable all on their own--they must be controlled.
Because of a natural tendency of processes to deteriorate over time, ongoing control is a necessity.

Stijloor.

Ronen E

Problem Solver
Staff member
Moderator
SPC = statistical process control

When we say a process in a state of statistical control or in control, it means the process is stable, predictable and consistent.

Here the word of "control" is obviously not the meaning of "exercise restraint" or "direction over"; or "dominate"; or "command". Then what is it?
In the context of "statistical process control" -

First of all, what is process control? In my understanding, it is ensuring that a process is consistently performing according to established desired characteristics (I believe this definition is broad enough, yet practical enough, to cover a majority of cases).

Statistical simply means that the above is achieved employing statistics (i.e. the multiple of statistic = A fact or piece of data derived from a large quantity of individual observations).

In the broader sense, I would say that control means ensuring that desired characteristics are consistently maintained over time.

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Bev D

Heretical Statistician
Staff member
Super Moderator
One more aspect: process control requires taking action when a process becomes unstable. In the days of Shewhart, action was taken based on the operator's beliefs; their signals were based on their experience or specification limits, or whatever. With statistical process control the signal to take action is based on a statistical assessement of wther or not the result that is observed is simple random variation or is beyond hwat would be expected.

So SPC is a statistical signal to take action - or not - to control the process...

J

junea9

Hey friend, this is very good thread. You have done really good work here.
Thanks for writing about the topic. This information is very useful for me.

A

ab001

We use a bus analogy at work (Who's driving this bus? Stop the bus I want to get off!)
Monitoring and Control:
Monitoring - your gauges that show speed, mileage, temp, check engine etc
Control - your steering wheel, foot pedals, indicators etc

Controls are the things you use to change the process- Hit the gas to speed up. Hit the brakes to slow down.

Process Control - commanding or directing the process to maintain output within desired limits (you decide what you want to see on the speedo)

Statistical Process Control - Process Control using statistics to monitor the output.

Steve Prevette

Deming Disciple
Staff member
Super Moderator
I have, over the years, seen many process indicators where there appears to be complete anarchy in the work group, but somehow the process results have settled into a stable and predictable (albeit with very high variability) results. Sort of proof of the "tribal knowledge" model versus management control.

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