Table.

Framework for assessing internal validity of articles dealing with prognosis^*

Study feature	Qualities sought
Sample of patients	Inclusion criteria defined, sample selection explained, adequate description of diagnostic criteria, clinical and demographic characteristics fully described, representative, assembled at common (usually early) point in course of disease, complete (all eligible patients included)
Follow up of patients	Sufficiently long
Outcome	Objective, unbiased (for example, assessment blinded to prognostic information), fully defined, appropriate, known for all or high proportion of patients
Prognostic variable	Fully defined, including details of measurement methods if relevant, precisely measured, available for all or high proportion of patients
Analysis	Continuous predictor variable analysed appropriately, statistical adjustment for all important prognostic factors
Treatment subsequent to inclusion in cohort	Fully described, treatment standardised or randomised

^*Partly based on references 9-13.