The papers by Best and Wake®eld, Fienberg et al. and Romaniuk et al. represent an interesting and stimulating set of somewhat diverse papers with some common themes, illustrating as expected the value of (and some of the problems encountered in) applications of multilevel and random-eects models over a wide range of medical and social science applications. More speci®c common themes shared by the ®rst two papers include a consideration of aspects of underascertainment of populations and/or cancer cases, and the use of Bayesian multilevel models.Best and Wake®eld's paper on accounting for inaccuracies in population and case counts in mapping studies expounds and illustrates the use of several errors-in-variables models, and in so doing again demonstrates the gain from the use of graphical models. This allows the population (and case) count models to be deconstructed, so that information on uncertainties about the sources of data can be incorporated explicitly, although the availability of such information is inevitably limited. Hence the sensitivity of geographical inferences about risks and relative risks of disease using conventional approaches to underenumeration and other errors is critically and eectively explored. Arguably, however, more details of the implementation of the Bayesian model are required for it to be fully assessable (or reproducible); some relevant recommendations for reporting such analyses are set out and illustrated elsewhere (Spiegelhalter et al., 1999) in the form of the BayesWatch checklist.Fienberg, Johnson and Junker begin with a succinct review of multiple-recapture methods, considering important extensions to allow heterogeneity of capture probabilities. Their paper is a tour de force which successfully draws together ideas from log-linear multiple-recapture models, Rasch models for object and list heterogeneity, and Bayesian hierarchical modelling approaches in addressing heterogeneity and dependence in capture±recapture modelling. Heterogeneity and dependence eects are apparently strong in the diabetes register and World Wide Web page examples investigated. The authors note the explicit simplicity of (complex) Rasch model formulation as hierarchical Bayes models. Although this makes constraints clear, as the authors also comment, the relationship between restrictions on interactions in log-linear and constraints on moments in Bayes models need further exploration before fully meaningful comparisons can be achieved.Romaniuk, Skinner and Cooper model diary data on shampoo usage. A multilevel logistic model is formulated in terms of binary usage responses, with levels identi®ed by survey respondents and by repeated hourly measurements throughout the diary week. This leads to problems in dealing with missing initial data, which are not really resolved by use of the EM algorithm. Although an alternative event history data formulation in terms of times between shampoo usage might reduce this problem, the apparent existence of strong patterns of usage by time of day and time since last ...