The International Collaborative Study involved a wide range of sample materials and ages and, on completion, assessed each stage independently (Scott et al 1989; Aitchison et al 1990). We combine here the three stages of the study and provide an overview of the uncertainties in the dating procedure as a whole and in its component processes. Three key optimal performance indices, related to internal and external precision and to bias, have been defined to allow quantitative assessment of Internal Consistency and External Consistency (Aitchison et al 1990). We believe that these measures provide quantitative descriptions of a laboratory's reproducibility, accuracy and precision.

For the internal consistency, we have defined the Internal Error Multiplier of the quoted error and, for the external consistency of any laboratory relative to an appropriate baseline, we have defined two indices, the Systematic Bias and External Error Multiplier of the quoted error. We have evaluated the three indices over the three stages and have assessed the relative performances of gas counting, accelerator and liquid scintillation laboratories. The quoted errors describe adequately the variability in duplicate results, but there is evidence of systematic biases and underestimation of interlaboratory variability. We have considered the contribution of pretreatment, synthesis counting to the overall variability for each laboratory type. We found that, for liquid scintillation (LS) and gas counting (GC) laboratories, ca 66% of the total variation is due to counting and sample synthesis whereas, for accelerator mass spectrometry (AMS) laboratories, the major sources of variability are the sampling and pretreatment processes.