1. Was a defined, representative sample of patients assembled at a common (usually early) point in the course of their disease?
  2. Was patient follow-up sufficiently long and complete?
  3. Were objective outcome criteria applied in a "blind" fashion?
  4. If subgroups with different prognoses are identified
    • Was there adjustment for important prognostic factors?
    • Was there validation in an independent group of "test-set" patients?

Reference: Evidence-based Medicine - Sackett, Richardson, Rosenberg and Haynes.