In an inter-laboratory comparison, different laboratories measure particular characteristics of a product. For cigarettes, smoke analyte yields in one or more homogeneous samples are measured under ISO-defined conditions. The comparison of different laboratories that perform comparable analyses with their own individual method is often called proficiency testing or laboratory performance studies. It is an essential part of the requirements for laboratory accreditation. Inter-laboratory comparisons are also used by groups intending to produce an analytical method to be used in a broader environment, such as international standards for a commercial interface, methods approved by regulatory bodies, official methods produced by reference committees or other types of international organisations. In this case the terms collaborative study or collaborative trial are currently used.

The required final results are the precision of the method in terms of repeatability and reproducibility. A sufficient number of participating laboratories is required to provide robust values. However, different organisations recommend different numbers of participating laboratories. For example ISO standards (ISO 5725-1, 1994) recommend 8-15 laboratories, IUPAC specify an absolute minimum number of 5 and AOAC recommend a minimum of 8. Different organisations also have different approaches towards the replication of experiments, for example, ISO does not specify the number of replicates.

Additional smoke constituent testing and regulation is likely in the future and further awareness and understanding of the statistical interpretation of generated data is required to facilitate testing based on sound scientific foundations. In this paper, we described the effects of the number of laboratories and replicates on the estimate of precision. We also examined the effect of the ratio SDR/SDr on the estimated confidence intervals in order to investigate the contribution of number of replicates on the reproducibility. We demonstrated that for a high ratio the minor improvement of within-laboratory precision, obtained by increasing the number of replicates, is totally diluted in the between-laboratory precision.