Table 1 Purpose categories: definitions and criteria of methodological rigor

Etiology (causation and safety)

Content pertains directly to determining if there is an association between an exposure and a disease or condition. The question is "What causes people to get a disease or condition?"

Observations concerned with the relationship between exposures and putative clinical outcomes; data collection is prospective; clearly identified comparison group(s); blinding of observers of outcome to exposure.

Prognosis

Content pertains directly to the prediction of the clinical course or the natural history of a disease or condition with the disease or condition existing at the beginning of the study.

Inception cohort of individuals all initially free of the outcome of interest; follow-up of at least 80% of patients until occurrence of a major study end point or to the end of the study; analysis consistent with study design.

Diagnosis

Content pertains directly to using a tool to arrive at a diagnosis of a disease or condition.

Inclusion of a spectrum of participants; objective diagnostic reference standard OR current clinical standard for diagnosis; participants received both the new test and some form of the diagnostic standard; interpretation of the diagnostic standard without knowledge of test result and vise versa; analysis consistent with study design.

Treatment

Content pertains directly to an intervention for therapy (including adverse effects studies), prevention, rehabilitation, quality improvement, or continuing medical education.

Random allocation of participants to comparison groups; outcome assessment of at least 80% of those entering the investigation accounted for in 1 major analysis at any given follow-up assessment; analysis consistent with study design.

Economics

Content pertains directly to the economics of a healthcare issue with the economic question addressed being based on the comparison of alternatives.

Question is a comparison of the alternatives; alternative services or activities compared on outcomes produced (effectiveness) and resources consumed (costs); evidence of effectiveness must from a study of real patients that meets the above-noted criteria for diagnosis, treatment, quality improvement, or a systematic review article; effectiveness and cost estimates based on individual patient data (micro-economics); results presented in terms of the incremental or additional costs and outcomes of one intervention over another; sensitivity analysis if there is uncertainty.

Clinical prediction guide

Content pertains directly to the prediction of some aspect of a disease or condition.

Guide is generated in one or more sets of real patients (training set); guide is validated in another set of real patients (test set).