Proficiency testing of sampling.

Proficiency testing of analytical laboratories is now ubiquitous-laboratories need to participate regularly to receive accreditation. The outcome helps participants to detect unexpected sources of uncertainty. But sampling, as well as analysis per se, introduces its own uncertainty in most types of analysis. Sampling uncertainty arises partly from heterogeneity in the 'target' (the particular mass of material for which the sample is a surrogate), but also from variation in the manner in which the sample is extracted. Different samplers will do the job somewhat differently, even when following a single protocol. The potential value of proficiency tests for sampling is therefore obvious. And in recent years, feasibility studies and schemes have appeared in several application sectors. Different sectors have very different constraints on how sampling and proficiency testing can be carried out. Nevertheless, the need for general guidelines is obvious.