Abstract

SUMMARY As part of a recently completed study of the effectiveness of breast cancer screening in Finland, 3975 women were invited to attend screens at 2-yearly intervals and the screens were classified as ‘favourable' or ‘unfavourable' in a simplification of the Wolfe classification of mammographic patterns. There is interest in both the rate of change from favourable to unfavourable states and vice versa, and the error rates involved in classifying patterns, in particular in whether these vary with age. We discuss simplifying assumptions which need to be made, because of the very short time series that is involved (four or five observations), to enable transitions between states to be disentangled from classification errors. A model for the data is proposed, the likelihood for the data given this model is obtained and a Markov chain Monte Carlo method is used to obtain posterior distributions. We show how our estimation procedure was checked in advance of the availability of the data, by means of a simulation.

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