Intersample variance

Precision is measured by making replicate measurements. As mentioned before, it is known to be a function of concentration and should be determined at the concentration level of interest. The intrasample variance can be determined by splitting a sample into several subsamples and carrying out the sample preparation/analysis under identical conditions to obtain a measure of RSD. For example, several aliquots of homogenized fish liver can be processed through the same extraction and analytical procedure, and the RSD computed. The intersample variance can be measured by analyzing several samples from the same source. For example, different fish from the same pond can be analyzed to estimate the intersample RSD. 

Wassenaar and Hobson [69] address the measurement of stable hydrogen isotope values in feathers from both captive and wild birds to determine the intra- and intersample variance. While the authors demonstrate the value of bulk stable isotope ratio measurements of feathers to assist in determining the origin of birds, the authors demonstrate the importance of determining the intra- and intervariances for a species of interest before geospatial determination of origin is attempted. 

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