To ensure the reliability of results, analytical laboratories require a continuous qualitycontrol program which must take account of both systematic and random errors . Analyses of reference materials can be used to estimate systematic errors but estimates of random errors (precision) tend to be optimistic, mainly because reference materials cannot be put through the whole analytical process (e .g ., primary sampling is often a major source of error) . Estimates of precision must be based on routine samples . If duplicate determinations are done on routine samples, the precision can be estimated reliably . Within the optimum concentration range of analytical methods (usually starting from 5-10 times the detection limit), the relative standard deviation (sr ) can be regarded as being almost constant or independent of concentration . The precision can then be estimated by first calculating the sr value of each pair of results . Individually, these are not reliable estimates of the true sr, but they can be regarded as independent measurements of the same sr and so can be pooled to obtain a more reliable estimate of precision with the number of duplicates as the degrees of freedom . The applicability of the method is tested on soil, rock and ore samples.