Reply to Collins and Le Manach

We thank Collins and Le Manach for their insightful comments on our paper and also for drawing our attention to their article on sample sizes for external validation of prognostic models, which unfortunately was unavailable when our data were analysed.
However, their article examined the effect of sample size for proportional hazards models and we used a logistic regression model (with previously published papers more applicable to the assessment of appropriate sample size).