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f •: r3e 4.2.1 PRECISION Precision of the data is a measure of the spread of the data when more than one mea- surement is taken on the same sample.. For duplicate measurements, precision can be expressed as the relative percent difference (RPD). A quantitative definition of the ,z RPD is given below: RPD = x2-X1 x 100 (xl+x2)!2 ' where: xl = first duplicate point value w x2 = second duplicate point value The sampling frequency for precision measurements will be at a minimum of 1 in 20 or �. I per set. r- 4.2.2 ACCURACY .- Feer Accuracy is an assessment of the closeness of the measured value to the true value. i Accuracy of chemical test re:alts is assessed by spiking samples with known standards and establishing the average ,recovery. In general, for organics, two types of recoveries are measured: matrix spike'recoveries and surrogate spike recoveries. For a matrix ��. spike, known amounts of standard compounds identical to the compounds present in the sample of interest are added to the sample. For a surrogate spike, the standards are chemically similar but not identical to the compounds in the fraction being ana- lyzed. The purpose of the surrogate spike is to provide quality control on every sample by constantly monitoring for unusual matrix effects and gross sample processing errors. Fer inorganics, only matrix spikes are measured in general. A quantitative definitic.rn of average recovery is given below. For spiked samples, the recovery (R) can be defined as a measure of accuracy: R = (Ci - c)/(C, x vx x 10-3) i ti. where: co = measured concentration analyzed in the sample without the addition of a spike (mfg) cs = concentration of standard added to the sample (mg/kg) vs = amount of standard added to the sample (mg/kg) sF0316041wp1004-51 4-2