The “cut-off probability” as the measure of quantal improbability in the dilution method

The MPN has been traditionally calculated on the basis of MPN tables of two different dilution schemes and thus the bacterial density estimation has been restricted to follow either of the above schemes. However, the use of a computer program may allow the use of any dilution scheme and at the same time make it unnecessary to ambiguously truncate the output result to three. On the other hand, it is well known that the MPN can be calculated with any output quantum irrespective of how improbable the quantum may be. Thus, the improbability should be estimated also in the computer work. In the present study, the direct use of “cut-off probability” as the measure of improbability is newly proposed on the basis that any quantum having maximum likelihood less than the probability can be regarded as improbable. The cut-off probability at the traditional erroneous rejection level of at most 1% was computed and then formulated into an equation applicable with some common dilution schemes. Furthermore, a static measure deviance was clarified to have limited availability and therefore be erroneous if used in general through comparison of the measure with the cut-off probability.