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to determine the source of error and provide for implementation of a corrective action. <br /> Affected sample results may be considered estimated or unusable during usability <br /> assessment. <br /> Surrogate Standard Evaluation. The results of surrogate standard determinations are <br /> compared with the true values spiked into the sample matrix before extraction and <br /> analysis, and the percent recoveries of the surrogate standards are determined. If these <br /> recoveries fall outside the control limits, the associated data may be affected. During the <br /> usability assessment, results may be considered as estimated or unusable based on the <br /> surrogate recoveries. <br /> Matrix Spike Evaluation. The observed recovery of the spike versus the theoretical <br /> spike recovery is used to calculate accuracy, as defined by the percent recovery. If the <br /> accuracy value exceeds the acceptance criteria for the given compounds, the sample <br /> set may need to be reanalyzed for the parameter in question, or the results considered <br /> as estimated. <br /> Additionally, blanks will be used to evaluate whether laboratory or field procedures <br /> represent a possible source of contamination in the field samples. Unmonitored <br /> contamination can allow false positive results to be reported and treated as true sample <br /> components, when in fact they are not; this type of error will adversely affect the <br /> accuracy of the reported results. <br /> A1.4.2 Precision <br /> Precision measures the reproducibility of repetitive measurements and is usually <br /> expressed in terms of imprecision. For this project, precision will be evaluated by <br /> determining the RPD in results for the MS/MSD and field duplicate samples, using the <br /> following formula: <br /> (C1 - C2) <br /> RPD = x 100 <br /> (Cl + C2)/2 <br /> Where: <br /> RPD: relative percent difference <br /> Cl: larger of the two observed values <br /> C2: smaller of the two observed values <br /> Imprecision in MS/MSD results is usually an indication of sample matrix effects whereas <br /> imprecision of field duplicate (collocated) sample results may be an indication of sample <br /> heterogeneity or multiplicative interferences that diminish or enhance analytical signals. <br /> Sample results that do not meet precision objectives may still be considered usable for <br /> Data Quality Objectives based upon professional judgment as to the cause and <br /> magnitude of the imprecision. All such decisions will be clearly justified in the data <br /> usability section of the validation report. <br /> Sampling Plan for Small Waste Disposal Areas (final).doc A-5 <br />