Determining the concentration of microorganisms controlled by attributes sampling plans

We present a method of translating the performance of attributes sampling plans, such as those recommended by the International Commission on Microbiological Specifications for Foods (ICMSF), so that the user can estimate the concentration of organisms that any plan may control. The method is based on the assumption that sampling results for organisms in foods are log-normally distributed. The normal distribution of log counts is used to estimate the proportion of defective (and marginally acceptable) material for a population of known (or, more usually, estimated) mean and standard deviation and then to determine the probability of acceptance. The method is presented in the form of a spreadsheet and is fully described and illustrated with examples of the performance of ICMSF plans in terms of concentrations of organisms controlled. The use of this approach to validate the use of sampling plans to meet a given performance criterion towards meeting a Food Safety Objective is also discussed.