Pre-Control Chart to guide in-process inspection / adjustment decisions


Don Watt

I've just inherited a new department and in order to try to start to move forward, I'm interested in trying to apply pre-control charting to guide in-process inspection / adjustment decisions.

I think it would suit an application where an operator is running a bank of 5 machines.
I'm told that we tried SPC ("didn't work - batch sizes too small / "Quality" depends on how good the set-up is")There also seems to be a mindset of if it's in tolerance it must be OK (reduction in variation is not viewed as continuous improvement - unfortunately)

Current inspection ssems to be a "finger in the air job" i.e. no data to support when to inspect / when to adjust.

To get to the point, does anybody know of a sampling plan to use a starting point, rather than trying to invent my own.


Rick Goodson


There are pre-control plans available but I would caution you that they all start with a caveat that the process first has to be in statistical control before you use the pre-control techniques. If you have not been able to implement SPC successfully it will be difficult to move to pre-control. A Cpk greater than 2.0 is not unusual as a minimum pre-control requirement.

Most plans are based on three zones. The green zone is 50% of tolerance and centered around the nominal dimension, the yellow zone adds another 25%, the balance is the red zone. There is a text, 'Modern Methods for Control and Involvement', ISBN 0-471-87696-X, with a brief discription of the technique or you might try papers by Dorian Shainin (1965) or N. R. Brown (1966). I also highly recommend you look at 'Statistical Quality Control' by Grant and Leavenworth, ISBN 0-07-024162-7. The authors present a number of reason why you should not use pre-control and in fact go so far as to state "The best advice we can give is to recommend almost any technique discussed in this book in preference to precontrol. The pattern of variation is a characteristic of the process and should never be confused with design tolerances."

Don Watt

Hi Rick,
Many thanks for the information, I'll certainly be having a look at the Grant & Leavenworth book.

I was under the impression that pre-control may be ok with Cp between 1 & 1.33 (the limited studies that I've done so far indicate that we have this basic capability)
If Cp of 2 or greater is required, I need to think again :-<

Jan Ketelaar

Hi Don,

I know what you are looking at. I also had the beauty of walking into a new department and it is still difficult to teach an old dog new tricks. I do work with a cp of 1-1.33 and it works okay. Soon I will start to introduce a system simular as the 50% / 25% zone. Only, I will use 1, 2 and 3 sigma. 2 Sigma being a yellow zone. If I have not been killed by then I will post the results....

Jim Jakosh

Pre Control Charts

What conditions have to to be in place to define "when" you can use a pre-control chart on an ongoing process?
Jim Jakosh_ Grand Rapids, Mi

Al Dyer


Can I assume that your currect process is in control?

Ifso, what would be the need or benefit of adding a pre-control chart?

Maybe some more details will help us!:)

Ravi Khare

PreControl will work well only when you already have a good Process Capability. To qualify a process, you will need five points in a row in the Green zone ( middle 50% of specs) on the precontrol chart. This would happen only if you have a good process capability with the process centered.

A precontrol chart would monitor whether your process maintains the low variation and holds on to the centre as you sample out of it.


Quite Involved in Discussions

I had seen a short article on this and was wondering if anyone could point me in a direction for some good resources on this? Thanks in advance!! :)

Mike S.

Happy to be Alive
Trusted Information Resource
If this zip file works okay here is a good article. Also, in Juran's Handbook version 4 there is a good write-up on it.


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