Accounting for bias due to a non-ignorable tracing mechanism in a retrospective breast cancer cohort study

Titman, Lancaster, Carmichael and Scutt have a new paper in Statistics in Medicine. This applies methods developed by Copas and Farewell (Biostatistics, 2001) to data from a retrospective cohort study where patients were traced conditional on survival up to a certain time. The authors note that the resulting observed process can be viewed as a purged process (Hoem, 1969). In addition to the pseudo-likelihood method of Copas and Farewell (which requires specification of the entry time distribution of patients), a full likelihood approach based on piecewise constant intensities under a Markov assumption is also applied. The term to take into account the conditional survival involves a transition probability, so estimation has similar difficulties to interval-censored data. For the breast cancer study considered, the two methods give very similar results.