[The yield of breast cancer screening: the importance of observational data and the problems of bias]. / De oogst van borstkankerscreening: het belang van observationele data en de problemen van bias.

Descriptive studies are important for the analysis of the effects of medical interventions. Especially in breast cancer screening, where the balance between gains and losses is often difficult to establish, detailed, empirical, long-term follow-up data on outcomes are of vital importance. Recent Dutch data comparing the prognosis, tumour stage at diagnosis and surgical treatment before and after the introduction of a population-based breast cancer screening programme reveal a better prognosis and a more favourable tumour stage after introduction of the programme. However, these data have their limitations. The interpretation of such figures is hampered by different types of bias (lead-time, length and over-diagnosis). The final assessment is always subjective and prone to confirmation bias.