Microbiological Sampling Plan Analysis Tool


Step 2b: Define a Sampling Plan: Concentration-Based plans

Concentration-based Sampling Plans

Concentration-based plans also require definition of the value “c”, but in this case it refers to the number of samples deemed marginally acceptable. This is termed the Acceptable Number of Concentrations within Limits; in a two-class plan any sample with a concentration higher than little-m (“m”) is marginally acceptable, whereas in a three-class plan marginally acceptable samples have the additional limitation of not exceeding big-M (“M”). Little-m is termed the Concentration Threshold for Marginal Acceptability and big-M is termed the Concentration Threshold for Unacceptability.  It is not necessary to include big-M in a concentration-based sampling plan.

If the calculation method used by the test is Direct Counting then the Analytical Sample Size must be provided. If instead the test relies on dilutions to estimate a Most Probable Number, the details of the dilutions must be entered as input.

As with Presence/Absence plans, Concentration-based plans must take into account the sensitivity of the analytical test method.  The input that defines this quality is either the Analytical Recovery Fraction, being the percent of existing organisms in the sample that are counted by the test, or the combination of Test Sensitivity and Count in Test Sample.

The Target Probability of Rejection 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 as the "Target".  Lots containing higher hazard concentration will have a rejection rate higher than the target. 


Percent of Lots Sampled, in percent

Number of Samples

Collected Sample Size, in g or ml

Either: Analytical Sample Size, in g or ml (for “Direct Counting”)

     Or: Number of Dilution Levels 
          and Number of Trials per Dilution Level 
          and Amount of Sample in the Lowest Dilution level, in g or ml (for “MPN”)

Either: Analytical Recovery Fraction, in percent

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

Acceptable Number of Concentrations within Limits

Concentration threshold for Marginal Acceptability, in cfu/g

Include Concentration Threshold for Unacceptability (check-box)

Concentration Threshold for Unacceptability, in cfu/g

Target Probability of Rejection, in percent

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< See Step 2a: Presence/Absence Plans                       See Step 3: Analysis and Reporting>

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