First of all, this is my first post on the Elsmar Cove forum. I hope this is not too wordy, I am probably breaking an etiquete rule.
Secondly, I am new to this tool called "pre-control." I come from the Deming school of quality. So, when you first look at it, it sounds like blasphemy. But, a co-worker engineer and I are currently working on implementing some "pre-control" tools, and I plan to post the results of this effort.
What about this debate?
I think that this is a fascinating debate about which is the better QC tool, "control charts" vs. "pre-control." I must say that Keki Bhote in "World Class Quality," chapters 20 to 22, makes an excellent case
against the "traditional control chart" method and
for the "pre-control" method. Bhote uses many examples to demonstrate the differences and advantages. He even takes actual examples and shows the data, probability and pitfalls where an operator using a control chart may think that the process is doing just fine and "in-control" when in fact he or she is making defectives with a poor Cpk of 0.51.
I tend to agree with Atul, the proof is in the experiment. So, I will need to perform some parallel process data collections with actual quality data transformed into both tools to examine and compare the two tools, take a look at the line stoppage and corrective action/fix cycles, etc.
Some key things that makes this debate intriguing:
The Deming group would argue that the "pre-control" method is not a clear view of the process variation and it promotes tampering with the system, making rules that the operator must "stop" to make an adjustment--tamper with the system. Deming would say that it invites tampering and does not at all equip the process operator with a tool that lets them continously control and reduce the process variation.
The Shainin-Bhote group would argue that "pre-control" method has enormous power in its simplicity and takes into account the product specification always. And, when the process is already established with a Cpk of 1.33 or better, than the b-risk of accepting bad product--drops to zero.
So which appears to be the better tool?
After reading this thread, and some of the literature behind this debate, I have concluded, for now...
A "traditional control chart" such as the Xbar & Rchart alone does not cut the mustard. If you want to use a traditional control chart, you would need to couple that tool with a running process capability curve-histogram and running Cpk values--a Cpk value that must be maintained daily with a set group of sample data, updating subgroup data regularly. This is not an easy tool to teach to the average machine operator with little to no statistical knowledge. Essentially, you would need to detect, stop and take action to fix that imaginary red-shaded part of the curve that goes over the upper spec. limit. Some software packages have this pictoral tool (curve & histogram) adjacent to the control chart.
The "pre-control" tool appears to be very simple in its "stop" and "go" rules and makes it simple for an operator to understand and follow. Five to six greens in a row assures a minimum Cpk of 1.33, for example.
In the coming weeks I plan to do some actual comparison tests to experiment and prove or disprove my thinking...this will take some time as we need to develop some software or find some that has the "pre-control" tool included. If anyone knows of a software package, let me know.