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CMAJ. 1995 February 1; 152(3): 351–357.
PMCID: PMC1337533

Basic statistics for clinicians: 3. Assessing the effects of treatment: measures of association.


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.

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Selected References

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  • Guyatt GH, Sackett DL, Cook DJ. Users' guides to the medical literature. II. How to use an article about therapy or prevention. A. Are the results of the study valid? Evidence-Based Medicine Working Group. JAMA. 1993 Dec 1;270(21):2598–2601. [PubMed]
  • Callahan CM, Dittus RS, Katz BP. Oral corticosteroid therapy for patients with stable chronic obstructive pulmonary disease. A meta-analysis. Ann Intern Med. 1991 Feb 1;114(3):216–223. [PubMed]
  • Stiegmann GV, Goff JS, Michaletz-Onody PA, Korula J, Lieberman D, Saeed ZA, Reveille RM, Sun JH, Lowenstein SR. Endoscopic sclerotherapy as compared with endoscopic ligation for bleeding esophageal varices. N Engl J Med. 1992 Jun 4;326(23):1527–1532. [PubMed]
  • Laird NM, Mosteller F. Some statistical methods for combining experimental results. Int J Technol Assess Health Care. 1990;6(1):5–30. [PubMed]
  • Oxman AD, Guyatt GH. A consumer's guide to subgroup analyses. Ann Intern Med. 1992 Jan 1;116(1):78–84. [PubMed]
  • Yusuf S, Wittes J, Probstfield J, Tyroler HA. Analysis and interpretation of treatment effects in subgroups of patients in randomized clinical trials. JAMA. 1991 Jul 3;266(1):93–98. [PubMed]
  • Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med. 1988 Jun 30;318(26):1728–1733. [PubMed]
  • Coldman AJ, Elwood JM. Examining survival data. Can Med Assoc J. 1979 Oct 20;121(8):1065–1071. [PMC free article] [PubMed]
  • Walter SD, Marrett LD, From L, Hertzman C, Shannon HS, Roy P. The association of cutaneous malignant melanoma with the use of sunbeds and sunlamps. Am J Epidemiol. 1990 Feb;131(2):232–243. [PubMed]
  • Forrow L, Taylor WC, Arnold RM. Absolutely relative: how research results are summarized can affect treatment decisions. Am J Med. 1992 Feb;92(2):121–124. [PubMed]
  • Naylor CD, Chen E, Strauss B. Measured enthusiasm: does the method of reporting trial results alter perceptions of therapeutic effectiveness? Ann Intern Med. 1992 Dec 1;117(11):916–921. [PubMed]
  • Bucher HC, Weinbacher M, Gyr K. Influence of method of reporting study results on decision of physicians to prescribe drugs to lower cholesterol concentration. BMJ. 1994 Sep 24;309(6957):761–764. [PMC free article] [PubMed]
  • Bobbio M, Demichelis B, Giustetto G. Completeness of reporting trial results: effect on physicians' willingness to prescribe. Lancet. 1994 May 14;343(8907):1209–1211. [PubMed]
  • Davey Smith G, Pekkanen J. Should there be a moratorium on the use of cholesterol lowering drugs? BMJ. 1992 Feb 15;304(6824):431–434. [PMC free article] [PubMed]
  • Guyatt GH, Sackett DL, Cook DJ. Users' guides to the medical literature. II. How to use an article about therapy or prevention. B. What were the results and will they help me in caring for my patients? Evidence-Based Medicine Working Group. JAMA. 1994 Jan 5;271(1):59–63. [PubMed]

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