In addition to categorization by methodology, studies can be characterized by content. Most articles under review can be placed into one of the following categories: evaluation of a new therapy, evaluation of a new diagnostic test, determination of the etiology of a condition or prediction of the outcome, or natural course of a condition. Each of these categories has different criteria for scientific review.

For a new therapy, some of the main issues include the following:

• Results - The magnitude of treatment effect and precision of the measurement

• Validity - Was there randomization into treatment groups, accounting for all study patients, blinding of participants and personnel, equality of treatment groups at baseline

• Impact - Applicability of results, benefits versus risk and cost

For a new diagnostic test, some of the main issues include the following:

• Test results - The presentation of likelihood ratios and the receiver operating characteristic (ROC) curve

• Validity - Was there blind and independent comparison to an accepted criterion ("gold") standard, was the test applied to a wide patient spectrum, what was the effect of performing the criterion standard, ease of test replication

• Impact - Patient applicability, effect of clinical management of the patient's condition, effect on patient care

For a study of the etiology of condition, some of the main issues include the following:

• Results - Strength of exposure to outcome, precise risk estimate

• Validity - Group similarity other than the variable (exposure) of interest, same exposure measurements, strong temporal relationship, adequate follow-up

• Impact - Results apply to patient population, magnitude of risk

For a study of the prediction of outcome, some of the main issues include the following:

• Results - Magnitude of outcome likelihood, precision of likelihood estimate

• Validity - Representative patient sample, sufficient follow-up, use of unbiased and objective outcome criteria

Impact - Equivalent patient population for comparison, effect on therapy choice