Estimation of sojourn time distributions and false negative rates in screening programmes which use two modalities.

Day and Walter derived methods of joint maximum likelihood estimation for the sojourn time distribution and the false negative rate for a screening programme. Their methods are not directly applicable to a programme which uses alternate screening by two modalities whose sojourn times and false negative rates will differ. A modification is proposed and the results applied to data from the Edinburgh Randomised Trial of Breast Cancer Screening. This enables the effects of mammography and clinical examination to be separated. It is estimated that in a programme using both modalities 79 per cent of tumours arising in regularly screened women would be detected by screening and if the clinical examination were omitted this figure would be reduced by 5 per cent. The confidence intervals are, however, quite wide.