The ideal OC curve would resemble Figure 11.5 with a vertical cut off at
the maximum acceptable level of defectives. To achieve this would require
testing an unacceptably high number of samples so the purchaser has to
adopt a sampling plan which will accept lots defined as being of good
quality most (e.g.95%) of the time and has a high probability (e.g.90%)
of rejecting lots of poor quality (Figure 11.6). Two types of error are
identifiable in this approach: the producer’s risk that a lot of acceptable
quality would be rejected (5%), and the consumer’s or purchaser’s risk that
a lot of unacceptable quality would be accepted (10%). Lots of interme-
diate quality would be accepted at a frequency of between 10 and 95%.
11.2.2 Three-class Attributes Plans
Three-class attributes sampling plans introduce a further category and
divide samples into three classes: acceptable, marginally acceptable, and
Figure 11.3 Operating characteristic curves (increasing stringency).c¼0,n¼3, 5, 10, 15
Figure 11.4 Operating characteristic curves (decreasing stringency).n¼5,c¼0, 1, 2, 3
402 Controlling the Microbiological Quality of Foods