The sampling model is designed to consider either presence/absence sampling plans or concentration-based plans.  Both two-class and three-class concentration-based plans can be defined.

A presence-absence plan determines lot acceptance or rejection based on testing of samples for the presence of any target organism.  A “positive” sample containing one microbe is operationally identical to a “positive” sample containing 1000 microbes. For this reason, presence-absence plans are most likely to be used in applications where the contamination is low enough that most or all of the samples would be expected to contain no organisms, and also in cases where the available enumeration procedures are deemed impractical for routine testing.

In a concentration-based plan, individual sample results are evaluated with respect to the estimated concentration of the organism in the sample taken. These types of plans are more appropriate for common microbes of low pathogenicity.

Both types of plans require values for the Number of Samples and the Analytical Sample Size (or a corresponding quality).  The tool also prompts for the Collected Sample Size, which is not used in calculation, and the Percent of Lots Sampled, which is used in risk reduction calculations when the plan is applied to a contamination profile with a between-lot distribution defined.
 

Presence/Absence Sampling Plans

These plans are defined by the Acceptable Number of Positive Samples, “c”, where any lot will be deemed acceptable if the number of samples testing positive are no more than “c”. This acceptance criteria is modified by the Probability of Detection, which is either provided directly as an entered percentage or estimated based on entered values for Test Sensitivity and Count in Test Sample.

Finally a Target Probability of Rejection is requested.  This value is used only in producing a value for the Detectable Microbial Load, that is, the arithmetic mean hazard concentration in a lot that will be rejected at the rate specified in the Target Probability of Rejection.  Lots containing a higher hazard concentration than this will have a rejection rate higher than the target.

REQUIRED INPUTS

Percent of Lots Sampled, in percent

Number of Samples

Collected Sample Size, in g or ml

Analytical Sample Size, in g or ml

Either: Probability of Detection, in percent

     or: Test Sensitivity, as a fraction of 1
          and Count in Test Sample, in log₁₀ cfu

Acceptable Number of Positive Samples

Target Probability of Rejection, in percent