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Tampering
Tampering with a process
occurs when we respond to variation In the process (such as by "adjusting"
the process) when the process has not shifted. In other words, it is when
we treat variation due to common causes as variation due to special causes.
This is also called "responding to a false alarm," since a false
alarm is when we think that the process has shifted when it really hasn't.
In practice, tampering
generally occurs when we attempt to control the process to limits that are
within the natural control limits defined by common cause variation. Some
causes of this include:
- We try to control
the process to specifications, or goals. These limits are defined externally
to the process, rather than being based on the statistics of the process.
- Rather than using
the suggested control limits defined at
 3
standard deviations from the center line, we instead choose to use limits
that are tighter (or narrower) than these (sometimes called Warning
Limits). We might do this based on the faulty notion that this will
improve the performance of the chart, since it is more likely that subgroups
will plot outside of these limits. For example, using limits defined
at 2
standard deviations from the center line would produce narrower control
limits than the 3
standard deviation limits. However, you can use probability theory to
show that the chance of being outside of a 3
standard deviation control limit for a Normally-distributed statistic
is .27% if the process has not shifted. On average, you would see a
false alarm associated with these limits once every 370 subgroups (=1/.0027).
Using 2
standard deviation control limits, the chance of being outside the limits
when the process has not shifted is 4.6%, corresponding to false alarms
every 22 subgroups!
Deming showed how tampering
actually increases variation. It can easily be seen that when we react to
these false alarms, we take action on the process by shifting its location.
Over time, this results in process output that varies much more than if
the process had just been left alone.
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