Requirements vs acceptability in proficiency testing schemes and other interlaboratory comparisons

Many laboratories take part in proficiency testing schemes, external qualit y assessment programmes and other interlaboratory comparisons. These have m any similarities but also important differences in their modus operandi and evaluation of performance of participating laboratories. This paper attemp ts to highlight both the similarities and differences. It also puts particu lar emphasis on requirements called "target values for uncertainty" and the ir meaning.