
Evaluating the validity of a Etiology/Harm study
Are the results of this article valid?COHORT Studies: Aside from the exposure of interest, did the exposed and control groups start and finish with the same risk for the outcome? 1. Were patients similar for prognostic factors that are known to be associated with the outcome (or did statistical adjustment level the playing field)? 2. Were the circumstances and methods for detecting the outcome similar? 3. Was followup sufficiently complete? CASECONTROL STUDIES: Did the cases and control group have the same risk (chance) of being exposed in the past? 1. Were cases and controls similar with respect to the indication or circumstances that would lead to exposure? 2. Were the circumstances and methods for determining exposure similar for cases and controls?
What are the results?How strong is the association between exposure and outcome? How precise was the estimate of the risk? Strength of inference: For 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) divided by c/(c + d) "RR of 3.0 means that the outcome occurs 3 times more often in those exposed versus unexposed." For casecontrol or retrospective studies:
Odds Ratio (OR) is the odds of previous exposure in a case divided by the odds of exposure in a control patient: OR = (a / c) divided by (b / d) OR = (exposed outcome yes / not exposed outcome yes) / (exposed outcome no / not exposed outcome no) "OR of 3.0 means that cases were 3 times more likely to have been exposed than were control patients." Confidence Intervals are a measure of the precision of the results of a study. For example, "36 [95% CI 2751]", a 95%CI range means that if you were to repeat the same clinical trial a hundred times you can be sure that 95% of the time the results would fall within the calculated range of 2751. Wider intervals indicate lower precision; narrow intervals show greater precision. Confounding Variable is one whose influence distorts the true relationship between a potential risk factor and the clinical outcome of interest. More information on odds ratios: The odds ratio Douglas G Altman & J Martin Bland BMJ 2000;320:1468 (27 May) How can I apply the results to patient care?Were the study subjects similar to your patients or population? Was the followup sufficiently long? Is the exposure similar to what might occur in your patient? What is the magnitude of the risk? Are there any benefits known to be associated with the exposure? Source: Guyatt, G. Rennie, D. Meade, MO, Cook, DJ. Users' Guide to Medical Literature: A Manual for EvidenceBased Clinical Practice, 2nd Edition 2008. Note: For criteria for other types of studies, see the following
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Revised July 2010 