🚀 go-pugleaf

> Date:          Mon, 6 Jul 1998 01:31:03 -0400
 ThomCycle@aol.com writes:

> > I've been going round and round with some colleagues about capability index
> > (Cpk) and their inherent  usefulness/unusefulness during early stages of
> > introduction of spc.  The thought are Cpk does nothing to give us unique
> > information about the process without the control chart and the histogram.
> > Can we abuse or overuse Cpk?  We have many other measures which give us
> > (worker, management, customers) information about the ability of the
 process,
> > do we need to add this tool in the early stages of our process management
> > implementation?  Do we need it internally if our customers aren't interested
> > in it?  I would like to here the opinion of others.
------------------
Generally speaking I say, "Yes,"  Cpk is a useful tool, even at the
early stages of an SPC initiative.  It can also be overused.  I have
found it useful to attempt to impress on people that a number is just
a number, and if you do not have the accompanying variables chart in
control over an extended period of time, you do NOT have a capability
index. If the chart is in control, you may have an index of: Short
Term Process potential; Long Term Process Potential; Short Term
Capability or Long Term Capability - depending on the period covered
by the supporting charts and some other things.  Be careful if
management starts demanding good Cpk's without understanding getting
processes in control is a full fledged undertaking in its own right.

If the chart is not in control, then at best you have simply a
snapshot in time of the behavior of your process during the time you
collected the supporting data..  All of that said, Cpk, regardless
how flaky can be useful.  It will tell you if re-centering your
process will help.     It can be translated to a parts per hundred
defective rate (or a ppm if you are fortunate) and management seems
to be able to understand that.  It may tell you that you are closer
to ppm than you thought, or that you have some real problems. I have
seen lots of cases of adequate Cp and Cpk during studies, and high
reject rates when going back to normal.  This indicated to me a lack
of discipline and other things which sometimes can be very important.

I don't know how much overlap there is with what Cpk can tell y'all
and what your existing reports can tell you - so in your case
capability indices may be redundant.

Hope this is helpful.

<< All opinions, statements, &c are my own>>
L.H. Garlinghouse, C.Q.E.
Pocahontas AR U.S.A.
(870) 892-4586  ext 7659
garlinghou@waterlooindustries.com