Re SPC Rules 4

[Short Ron <ron.short@marshalls.co.uk>]

I have read with great interest the topic on SPC and at last I feel we have
had a common sense reply/
I feel if a process is totally unstable then we need to step back Think and
take action.
Then we use SPC to determine if what we have done has lead to an
improvement.
We then continue with a control chart to constantly monitor the variation,
assuming if we set ourselves a upper and lower limit that is acceptable to
our process.
If at some point the process goes out of control SPC will alert us of this
fact.
I have made an alteration to a batch weighing process which at the onset was
anything up to 15 kgs above its set point.
I didn't need a control chart to tell me this.
I took action and have now got the process stable within -1 to + 2 kg.
Out of 20 data points I may find that at the most 1 point goes 1 kg outside
the UCL or LCL but the remaining  points are within limits.
The outliers I suppose could be classed as a special cause but I choose not
to class them as special or common as in the past corrective action on 1 or
2 outliers can have a detrimental effect on the remainder.
In short I agree with the posting that use of control charts allows you to
make better decisions within the process. Also assuming because of the use
of SPC you have gained a understanding of what you wish to achieve then you
will have also gained the knowledge to use your own judgement re the special
causes or outliers.
Ron Short