7.2.2 Decision Mechanism 2: Comparison of the Mean of Replicate Data from the DU to the Action Level
Comparison of the mean of replicate ISM sample results to an action level is most useful when quantifying the uncertainty in the mean is not an important element to the decision.In this decision mechanism, replicate ISM samples are collected in the field from the same DU. These replicates provide a measure of the variability of the entire sampling, preparation, and analytical process. The mean concentration of the replicates is calculated and compared to the action level. The mean concentration from replicate samples is likely to be closer to the true mean of the DU than the result from a single ISM sample (see Section 4) and could therefore be considered to provide a more reliable estimate of the mean. There is no assurance, however, that the actual mean concentration has not been underestimated. Consequently, this decision mechanism would not be useful in circumstances where project objectives dictate that estimates of mean concentrations must be conservative (e.g., EPC values in most USEPA human health risk assessments).
Decision Mechanism 2 example
This example is similar to the one provided for Decision Mechanism 1, except three replicate samples are collected from the DU. The reported concentrations of PCBs in the replicates are 0.12, 0.16, and 0.26 mg/kg. The mean concentration of PCBs based on the three replicate samples is 0.18 mg/kg. This does not exceed the action level for residential land use of 0.22 mg/kg, so it appears that no further action may be warranted. Note, however, how close the estimate of the mean is to the action level. This fact may have important implications for decision making. This same example data set is used again in the example for Decision Mechanism 3 to further illustrate these implications.