unacceptable. Use of this extra classification of marginally acceptable
means that they are not used with presence or absence tests but only with
microbiological count data. A three-class plan is defined by four numbers:
n – the number of samples to be taken from a lot;
M– a count which if exceeded by any of the test samples would lead to
rejection of the lot;
m – a count which separates good quality from marginal quality and
which most test samples should not exceed;
c – the maximum number of test samples which may fall into the
marginally acceptable category before the lot is rejected.
As with the two-class plan increasingcor decreasingn increases the
leniency of a three-class plan. OC curves for three-class plans can be
derived from the trinomial distribution and describe a 3-dimensional
Figure 11.5 An ideal operating characteristic curve
Figure 11.6 Producer’s and consumer’s risk
Chapter 11 403