
564 A First Course in Quality Engineering
A (“large”) sample is taken from the process and the proportion defectives
in the sample are calculated� The sigma level of the process is equated to the
sigma level of a normal process producing the same level of defectives as in
the sample data� Table 9�1 facilitates reading the sigma level of a process given
the proportion defectives in the sample� For example, if the sample data for
an attribute showed that there were 6200 defects per million (0�62%), then the
quality level of the process is 4σ, because, a normal process with 4σ capability
will produce 0�62% outside specication in the worst con ...