Strengths and limitations
illness
The differences in clinical characteristics between the observational and the trial data participants was expected, but cannot be attributed just to clinical decision making in deciding who to recruit to the trial or the observational study as some observational patients came from less typical sites that were not able to recruit to the trial (for example, A&E departments). The prognostic model was limited by the relatively few children where illness progressed (potentially limiting detection of relevant variables because of lower power), but using both traditional rules of thumb and more recent guidance the study should have had sufficient power, at least for the three-item model.The discrimination of the model was good, but the current study may have underestimated the discriminatory ability, as more significant variables would increase the discriminatory power. Bootstrapping limited the problem of overfitting and provided internal validation, which is the recommended most-efficient approach to internal validation rather than using split samples.