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Reference or Check Standard Evaluation. Standard reference materials analyzed by the laboratory following U.S. EPA method protocols (e.g., laboratory control sample [LCS], continuing calibration [CC]) are compared with true values and acceptable ranges as indicators of accuracy. Values outside the acceptable ranges require corrective action to determine the source of error and provide for implementation of a corrective action. Affected sample results may be considered estimated or unusable during usability assessment. Surrogate Standard Evaluation. The results of surrogate standard determinations are compared with the true values spiked into the sample matrix before extraction and analysis, and the percent recoveries of the surrogate standards are determined. If these recoveries fall outside the control limits, the associated data may be affected. During the usability assessment, results may be considered as estimated or unusable based on the surrogate recoveries. - Matrix Spike Evaluation. The observed recovery of the spike versus the theoretical spike recovery is used to calculate accuracy, as defined by the percent recovery. If the accuracy value exceeds the acceptance criteria for the given compounds, the sample set may need to be reanalyzed for the parameter in question, or the results considered as estimated. Additionally, blanks will be used to evaluate whether laboratory or field procedures represent a possible source of contamination in the field samples. Unmonitored contamination can allow false positive results to be reported and treated as true sample components, when in fact they are not; this type of error will adversely affect the accuracy of the reported results. A1.4.2 Precision i Precision measures the reproducibility of repetitive measurements and is usually expressed in terms of imprecision. For this project, precision will be evaluated by determining the RPD in results for the MS/MSD and field duplicate samples, using the following formula: (Cl-C2) RPD = x 100 (Cl + C2)/2 j Where: RPD:relative percent difference Cl:larger of the two observed values C2:smaller of the two observed values jImprecision in MS/MSD results is usually an indication of sample matrix effects whereas imprecision of field duplicate (collocated) sample results may he an indication of sample -a