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Field duplicates can either be submitted blind or known to the lab.  With a field duplicate a second sample is collected at the same time and location in separate containers.  Samples are homogenized where possible in the field (i.e. alternate filling of sample and duplicate for water and bowl mixing for soils). Note that VOCs should not be homogenized. Field duplicates evaluate analytical precision, field precision and sample homogeneity and are of limited use for samples that cannot be homogenized. Meaningful RPDs for field duplicates can be calculated using the same procedures that are used to calculate lab duplicate RPDs. Field duplicate data may not match the sample results for a variety of reasons, other than analytical issues, including, results of both are near the method RDL, field duplicate and sample are not equally homogenized in the field before dividing between containers. Some matrices are difficult to homogenize such as rocky soils. The RPD of the sample and the field duplicate are very dependent on how they are collected and homogenized in the field.