A bootstrap sample usually means that a random sample is drawn with replacement from the original sample. This bootstrap sample includes the same number of patients (n) as in the original sample, but some are excluded, others included once, twice, etc. The model is developed in the bootstrap sample, and validated in the original sample. The difference indicates the optimism in model performance. This procedure is efficient, since n patients are used for development and n for validation. To obtain stable results, the procedure has to be repeated, usually at least 100 times .