There are two general ways to compare a treatment and a control group, relative comparisons and absolute comparisons. For a relative comparison, the basic computation is division. When the ratio A/B is larger than one, that implies that A is superior to B. For an absolute comparison, the basic computation is subtraction. When the difference A-B is greater than zero, that implies that A is superior to B. The distinction between relative and absolute is not the only important distinction, but it does represent a fundamental property of measures of effectiveness. Relative measures represent measures where the fundamental calculation is a division and absolute measures represent measures where the fundamental calculation is a subtraction.
Surprisingly, the choice between division and subtraction is critical. Relative and absolute comparisons often paint quite a different picture. There is substantial consensus in the evidence-based medicine community that relative measures of risk (such as an odds ratio or relative risk) tend to be misperceived by clinicians and patients and that they get a better sense of the value of a treatment when they receive information presented on an absolute scale (such as an absolute risk reduction or a number needed to treat). The general trend is that interventions appear more attractive when presented in relative terms, but less attractive when presented in absolute terms. The effect is strongest when the events being compared are relatively rare.
- Completeness of reporting trial results: effect on physicians' willingness to prescribe. Bobbio M, Demichelis B, Giustetto G. Lancet 1994: 343(8907); 1209-11. [Medline]
- General practice registrar responses to the use of different risk communication tools in simulated consultations: a focus group study. Edwards A. British Medical Journal 1999: 319(7212); 749-752. [Medline] [Abstract] [Full text] [PDF]
- Explaining risks: turning numerical data into meaningful pictures. Edwards A, Elwyn G, Mulley A. Bmj 2002: 324(7341); 827-30. [Medline] [Full text] [PDF]
- evidence-based purchasing: understanding results of clinical trials and systematic reviews. Fahey T, Griffiths S, Peters TJ. British Medical Journal 1995: 311(7012); 1056-9; discussion 1059-60. [Medline] [Abstract] [Full text]
- Absolutely relative: how research results are summarized can affect treatment decisions. Forrow L, Taylor W, Arnold R. The American Journal of Medicine 1992: 92(2); 121-24.
- Communicating the benefits of chronic preventive therapy: does the format of efficacy data determine patients' acceptance of treatment? Hux J, Naylor C. Medical Decision Making 1995: 15(2); 152-7.
- Absolute and relative truth in clinical trials. Julian D. Lancet 2002(June): 359(9321); 1945-1946. [Medline] [Full text]
- Quality of life questionnaires: does statistically significant = clinically important? Juniper EF. J Allergy Clin Immunol 1998: 102(1); 16-7. [Medline] [Full text] [PDF]
- An assessment of clinically useful measures of the consequences of treatment. Laupacis A, Sackett D, Roberts R. New England Journal of Med 1988: 318(26); 1728-1733. [Medline]
- Consider absolute risks in SIDS prevention. Logan S, Bedford H, Elliman D. Arch Dis Child 2000: 83(5); 457. [Medline] [Full text] [PDF]
- Who benefits from medical interventions? Smith GD, Egger M. Bmj 1994: 308(6921); 72-4. [Medline] [Full text]
- “Absolute” is inappropriate for quantitative risk estimation. Tunstall-Pedoe H. BMJ 2000: 320(7236); 723-. [Full text]
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