MUSM Libraries: Evaluating a HARM/ETIOLOGY article

Appraise |  Strength of inference | Finding articles


What did they do? (Methods)

1. Were there clearly defined groups of patients, similar in all important ways other than exposure to the treatment or other cause?

2. Were the treatments/exposures and clinical outcomes measured in the same ways in both groups? (Was the assessment of outcomes either objective or blinded to exposure?

3. Was the follow-up of the study patients sufficiently long (for the outcome to occur) and complete?

4. Do the results of the harm study fulfill some of the diagnostic tests for causation?

5. Were the outcomes and exposures measured in the same way in the groups being compared?

What was the answer? (Results)

1. How strong is the association between exposure and outcome?

2. How precise is the estimate of risk?

What did they say about the answer? (Conclusion)

1. Do the results of the study support the author/s conclusions?

2. Does the conclusion reflect the discussion?

3. Are there study limitations, and do these impact the conclusions?

4. Are potential discrepancies mentioned?

What do I do with this information?

1. Were the study patients similar to the patient in my practice?

2. Was the duration of follow-up adequate?

3. What was the magnitude of the risk?

4. Should I attempt to stop the exposure?

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Strength of inference

Exposed Yes a b
Exposed No c d

RCT or Prospective cohort studies:

Case-control studies:

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

Number Needed to Treat (NNT) The number of patients that need to be treated with a specified therapy in order to prevent one additional bad outcome. Calculated as the inverse of the absolute risk reduction (1/ARR)

Absolute Risk Reduction (ARR) is the difference in risk between the control group (X) and the treatment group (Y). ARR = X-Y

Control Event Rate (CER)
The proportion of patients in the control group who experience the studied event.

Experimental Event Rate (EER)
The proportion of patients in the experimental treatment group who are observed to experience the outcome of interest.

Relative Risk Reduction (RRR) is the percent reduction in risk in the treated group (Y) compared to the control group (X). RRR = 1-RR x 100%

Experimental group size:
Control group size:
Events in experimental group:
Events in control group:


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Finding articles about harm/etiology



From: Guyatt, G. Users' Guides to the Medical Literature: Essentials of Evidence-based Clinical Practice. AMA Press, 2002 and Strauss. Evidence-Based Medicine. How to Practice and Teach EBM. Churchill-Livingstone, 3rd edition, 2005 (pocket cards).

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