Users' Guides to the Medical Literature

Evidence-Based Medicine Working Group

Are the Results Valid?

Except for the exposure under study, were the compared groups similar to each other?

Were the outcomes and exposures measured in the same ways in both groups?

Was the follow-up of patients sufficiently long and complete?

Is the temporal relationship correct?

Is there a dose-response gradient?

What Are the Results?

How strong is the association between exposure and outcome?

How precise is the estimate of risk?

Will the Results Help Me in My Patient Care?

Are the results applicable to my patients?

What is the magnitude of the risk?

Should I attempt to stop the exposure?

Reference:

JAMA 1994; 271:1615-1619

Strength of Inference:

RCT > Cohort > Case-Control > Case Series

Outcome + | Outcome - | |

Exposure + | a | b |

Exposure - | c | d |

From RCT or prospective cohort studies, Relative Risk (RR) is the risk of the outcome in the exposed group divided by the risk of the outcome in the unexposed group:

RR = a/(a+b) / c/(c+d) "RR of 3.0 means that the outcome occurs 3 times more often in those exposed versus unexposed"

From case-control studies, Odds Ratio (OR) is the odds of exposure in a case divided by the odds of exposure in a control patient:

OR = (a/c) / (b/d) = ad/bc "OR of 3.0 means that cases were 3 times more likely to have been exposed than were control patients"

A Confounding Variable is one whose influence distorts the true relationship between a potential risk factor and the clinical outcome of interest

Finding An Article About Harm or Etiology

Risk (tw)

Explode Cohort Studies

[see ACP J Club 1994 Nov-Dec; A10 - A11]

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