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Supplement to Chapter 17 Statistical process control (SPC) 523<br />

Figure S17.3 Process capability compares the natural variation of the process with the specification range which<br />

is required<br />

Worked example<br />

In the case of the process filling boxes of rice, described previously, process capability can<br />

be calculated as follows:<br />

Specification range = 214 − 198 = 16 g<br />

Natural variation of process = 6 × standard deviation<br />

= 6 × 2 = 12 g<br />

C p = process capability<br />

UTL − LTL<br />

=<br />

6s<br />

214 − 198<br />

= =<br />

6 × 2<br />

= 1.333<br />

If the natural variation of the filling process changed to have a process average of 210 grams<br />

but the standard deviation of the process remained at 2 grams:<br />

214 − 210 4<br />

C pu = = = 0.666<br />

3 × 2 6<br />

210 − 198 12<br />

C pl = = = 2.0<br />

3 × 2 6<br />

C pk = min(0.666, 2.0)<br />

= 0.666<br />

16<br />

12

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