Re: Prediction

["Dr A D Burns" <aburns@q-skills.com>]

John,

If you have a process where points are outside control limits, the process
is "out of control" and is therefore unpredictable.  There is no point in
trying to estimate how unpredictable it is compared to another out of
control process.  It is best to first work on the most upstream process and
bring it to an "in control" state.

Wherever possible use raw, variable data (measurements) because this
contains the most information about the process.  The purpose of control
charts is to help understand what the process is doing.  Counting the number
of times a variable goes outside control limits tells very little about the
process.  Only use "defect" counts where there is no alternative, such as
the number of blemishes on an item, that is, where a blemish exists or it
doesn't.  Do not use targets of zero defects or other levels of defects
(Deming's point 10) ... they achieve nothing.  The number of defects will
always depend on where you set specification limits ... you can achieve any
defect level you want simply by changing spec limits.

Dr Tony Burns
aburns@q-skills.com