Tampering

adjusting a stable process and making it worse

Tampering is reacting to every deviation as if it were a special cause, for example adjusting the machine after each measurement that is off nominal. In a stable process this increases the variation instead of reducing it; Deming’s funnel experiment shows an increase of about 41 % with the simplest adjustment rule.

The cure is to adjust only on a real signal from a control chart.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Common cause: natural, random variation
• Stable process: only common-cause variation
• Control limits: voice of the process