Document Detail

Basic statistics for clinicians: 3. Assessing the effects of treatment: measures of association.
MedLine Citation:
PMID:  7828099     Owner:  NLM     Status:  MEDLINE    
In the third of a series of four articles the authors show the calculation of measures of association and discuss their usefulness in clinical decision making. From the rates of death or other "events" in experimental and control groups in a clinical trial, we can calculate the relative risk (RR) of the event after the experimental treatment, expressed as a percentage of the risk without such treatment. The absolute risk reduction (ARR) is the difference in the risk of an event between the groups. The relative risk reduction is the percentage of the baseline risk (the risk of an event in the control patients) removed as a result of therapy. The odds ratio (OR), which is the measure of choice in case-control studies, gives the ratio of the odds of an event in the experimental group to those in the control group. The OR and the RR provide limited information in reporting the results of prospective trials because they do not reflect changes in the baseline risk. The ARR and the number needed to treat, which tells the clinician how many patients need to be treated to prevent one event, reflect both the baseline risk and the relative risk reduction. If the timing of events is important--to determine whether treatment extends life, for example--survival curves are used to show when events occur over time.
R Jaeschke; G Guyatt; H Shannon; S Walter; D Cook; N Heddle
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't    
Journal Detail:
Title:  CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne     Volume:  152     ISSN:  0820-3946     ISO Abbreviation:  CMAJ     Publication Date:  1995 Feb 
Date Detail:
Created Date:  1995-02-22     Completed Date:  1995-02-22     Revised Date:  2009-11-18    
Medline Journal Info:
Nlm Unique ID:  9711805     Medline TA:  CMAJ     Country:  CANADA    
Other Details:
Languages:  eng     Pagination:  351-7     Citation Subset:  AIM; IM    
Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ont.
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MeSH Terms
Case-Control Studies
Data Interpretation, Statistical*
Odds Ratio
Survival Analysis
Treatment Outcome*
Erratum In:
Can Med Assoc J 1995 Mar 15;152(6):813

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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