Evaluation (or assessment)–time bias can arise in oncology trials that study progression-free survival (PFS) when randomized groups have different patterns of timing of assessments. Modelling or computer simulation is sometimes used to explore the extent of such bias; valid results require building such simulations under realistic assumptions concerning the timing of assessments. This paper considers a trial that used a logrank test where computer simulations were based on unrealistic assumptions that severely overestimated the extent of potential bias. The paper shows that seemingly small differences in assumptions can lead to dramatic differences in the apparent operating characteristics of logrank tests. Copyright © 2010 John Wiley & Sons, Ltd.