Mathematical treatment of plates with colony counts outside the acceptable range.

The exclusion of plate counts outside of an acceptable range can bias the estimation of concentration. If these plates are too numerous to count (TNTC), then their microbes may be inhibited. If inhibition of the microbes on a plate is suspected, then its count may be best treated as a lower bound. When these lower bounds replace counts for some plates, the estimate and confidence interval must be calculated accordingly. Also, a measure of unusualness for plate counts is discussed.