by accuracy? Do population differences in dental maturity infl uence accuracy? There is an urgent need for an evidencebased reference to address some of these questions. The fi rst aim of this study was to calculate the bias (difference between dental and real age) of age-estimating methods that use mandibular third molar (M3) root formation. The probability of being at least 18 given M3 root stage is also of interest. The second aim was to apply diagnostic tests of accuracy showing how root stage discriminates between individuals at least 18 years of age and those younger and to apply this knowledge to predict the likelihood of age 18 for a single individual. For this part of this study we used a separate reference sample of 1,663 radiographs. We propose an age interval for M3 root stages to aid interpretation of the term 'on the balance of probabilities' . We highlight the similarity in M3 apex maturity between world groups from published data and illustrate how a small group difference in average age has little impact on the confi dence interval of estimated age for an individual.

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