Inclusion Criteria

More than 130 journals are reviewed to identify articles that meet our criteria for scientific merit, as set out below:

Basic criteria for original or review articles:

• in English.
• about adult humans.
• about topics that are important to the clinical practice of general practice/family practice, general internal medicine and its subspecialties other than descriptive studies of prevalence.
• analysis of each article is consistent with the study question.

Studies of prevention or treatment must meet these additional criteria:

• random allocation of participants to comparison groups.
• follow-up (endpoint assessment) of at least 80% of those entering the investigation.
• outcome measure of known or probable clinical importance.

• inclusion of a spectrum of participants, some but not all of whom have the disorder or derangement of interest.
• objective diagnostic ("gold") standard (e.g., laboratory test not requiring interpretation) OR current clinical standard for diagnosis (e.g., a venogram for deep venous thrombosis), preferably with documentation of reproducible criteria for subjectively interpreted diagnostic standard (i.e., report of statistically significant measure of agreement beyond chance among observers).
• each participant must receive both the new test and some form of the diagnostic standard.
  interpretation of diagnostic standard without knowledge of test result.
• interpretation of test without knowledge of diagnostic standard result.

Studies of prognosis must meet these additional criteria:

• inception cohort of individuals, all initially free of the outcome of interest.
• follow-up of at least 80% of patients until the occurrence of a major study endpoint or to the end of the study.

Studies of etiology must meet these additional criteria:

• exploration of the relation between exposures and putative clinical outcomes.
  
• prospective data collection with clearly identified comparison groups for those at risk for the outcome of interest (in descending order of preference from randomized controlled trial, quasi-randomized controlled trial, nonrandomized controlled trial, cohort studies with case-by-case matching or statistical adjustment to create comparable groups, to nested case-control studies.
  
• masking of observers of outcomes to exposures (criterion assumed to be met if outcome is objective, i.e., all-cause mortality, objective test).

Studies of quality improvement or continuing education must meet these additional criteria:

• random allocation of participants or units to comparison groups.
  
• follow-up of at least 80% of participants.
  
• outcome measure of known or probable clinical or educational importance.

Studies of the economics of health care programs or interventions must meet these additional criteria:

• the economic question addressed must be based on comparison of alternatives in real patients.
  
• alternate diagnostic or therapeutic services or quality improvement activities must be compared on the basis of both the outcomes produced (effectiveness) and resources consumed (costs).
  
• evidence of effectiveness must be from a study (or studies) of real (not hypothetical) patients, which meets the above-noted criteria for diagnosis, treatment, quality improvement, or a systematic review article that also meets criteria.
  
• results should be presented in terms of the incremental or additional costs and outcomes of one intervention over another.
  
• where uncertainty exists in the estimates or imprecision in the measurement, a sensitivity analysis should be done.

Studies of clinical prediction guides must meet these additional criteria:

• the guide must be generated in one or more sets of real (not hypothetical) patients (training set).
  
• the guide must be validated in another set of real (not hypothetical) patients (test set) and must deal with treatment, diagnosis, prognosis, or etiology.
  

Studies of differential diagnosis must meet these additional criteria:

• a cohort of patients who present with a similar, initially undiagnosed but reproducibly defined clinical problem.
  
• clinical setting, including the referral filter, is explicitly described.
  ascertainment of diagnosis for 80% of patients using a reproducible diagnostic workup strategy for all patients and follow-up until patients are diagnosed or follow-up of 1 month for acute disorders or 1 year for chronic or relapsing disorders.

Systematic review articles must meet these additional criteria:

• an identifiable description of the methods indicating the sources and methods for searching for articles.
  
• statement of the clinical topic and the inclusion and exclusion criteria for selecting articles for detailed review.
  
• at least one article in the review must meet the above noted criteria for treatment, diagnosis, prognosis, clinical prediction, etiology, quality improvement, economics of health care, or differential diagnosis.

These criteria are subject to modification if, for example, it is found feasible to apply higher standards that increase the validity and applicability of studies for clinical practice. The objective of the criteria screen is to include only the very best literature, consistent with a reasonable number of articles "making it through the filter."