A First Course in Quality Engineering, 2nd Edition

272 A First Course in Quality Engineering

Figure 4�18 has been drawn to illustrate the meaning of this index� Notice

how, by comparing the distance of the nearest specication from the process

center with 3σ, the index evaluates whether the part of the process which

is most likely to exceed the specication does in fact exceed it� If the value

of C

pk

is less than 1�0, the process is producing values outside specication�

The larger the value of C

pk

, the better the process is in meeting specication�

Again, when we use estimates for μ and σ in the above formula, we only get

an estimate for C

pk

� We have to use large samples (n ≥ 100) to mini