| Were there clearly defined groups of patients, similar in all important ways other than exposure to the treatment or other cause? |
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| Were treatments/exposures and clinical outcomes measured in the same ways in both groups (was the assessment of outcomes either objective or blinded to exposure)? |
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| Was the follow-up of study patients complete and long enough? |
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| Is it clear that the exposure preceded the onset of the outcome? |
| Is there a dose-response gradient? |
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| Is there positive evidence from a "dechallenge-rechallenge" study? |
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| Is the association consistent from study to study? |
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| Does the association make biological sense? |
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In a randomised trial or cohort study: relative risk = RR = {a/(a+b)}/{c/(c+d)}
In a case-control study: odds ratio (or relative odds) = OR = ad/bc
