Dictionary of Evidence-based Medicine.pdf

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Dictionary of Evidence-based Medicine 59 Excess risk In a comparative study of risks associated with exposure to two therapies or environments, the difference in risk can be expressed in absolute or relative terms (see Risk difference, Relative risk and Risk ratio). The ratio of the risk associated with one exposure divided by the risk associated with the other is the relative risk. Suppose that the risk of developing diarrhoea after receiving a course of antibiotic A is 5 in 100 and 2 in 100 for antibiotic B, then the relative risk of developing this side-effect when receiving A instead of B is 2.5 (i.e. 0.05/0.02). The excess risk is the difference in risk expressed as a percentage or 150% in this case (i.e. 100 x (5-2)/2). A common mistake in the literature is to interpret a risk ratio or relative risk of 2.5 as a 250% or two and a half-fold increase in risk (Lam TH (1997) Relative risks are inflated in published literature. BMJ. 315: 880). Explanatory trial analysis (see under Intention to treat analysis) Exponential survival model The exponential survival model is characterized by the following survival function, where T is a random variable representing failure time: S(t) = P(T ^ t) = exp(-to) The corresponding probability density function is given by/(y) = A exp(-Af). The mean (j, of the exponential distribution is given by I/A and its variance a 2 by I/A, 2 . The shape of the survival curve is of the type shown in Figure 8. Expressed preference method (see under Willingness to pay) External validity External validity refers to how applicable the results of a study are to the target population. For example, a frequently expressed concern is whether conclusions drawn from the results of clinical trials using patients adhering to strict inclusion criteria are applicable to the population of patients visiting primary care physicians. In other words, critics question the external validity or generalizability of such clinical trial data to the average primary care patient.
60 Dictionary of Evidence-based Medicine Figure 8 Exponential survival model Externality Allocative efficiency occurs when the marginal benefits equal the marginal costs. In a number of instances, the marginal private cost (that which the consumer pays) does not fully reflect the marginal cost to society. A classic example arises when manufacturing of a particular product causes pollution. The additional cost of cleaning up the environment is not usually reflected in the market price of the product and can hence be viewed as the subsidy provided by society to the consumer of the product. Such costs are referred to as external costs or externalities. In the presence of a negative externality (E), a quantity (Ql) which is more than the optimum quantity (QO) of the product is produced and the resources are not allocated efficiently (Figure 9). Externalities can be positive too. For example, by vaccinating a child, not only is the child better protected from the potential invading micro-organism but society as a whole benefits from lower transmission rates of the infection through the development of herd immunity. It has also been argued that human beings generally do not like to see others suffer from untreated illness and that they care for the welfare of others. Therefore, there is associated with health care a positive caring externality (Culyer AJ (1980) The political economy of social policy. Martin Robertson, Oxford).
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Dictionary of Evidence-based Medici
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Preface This book was conceived at
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A Ability to pay Ability to pay is
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Figure I Audit: Clinical audit cycl
Page 14 and 15: B Balanced design In an experiment,
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Page 72 and 73: G Game theory Game theory, first de
Page 86 and 87: I ICD (see International Classifica
Page 94 and 95: K Keynes, John Maynard (1883-1946)
Page 100 and 101: M Macroeconomics Macroeconomics is
Page 112 and 113: N N of I trial An N of 1 trial is a
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o OBM (see Opinion-based medicine)
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Figure 25 Measuring quality of life
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S Scarcity Scarcity is often referr
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T TDM (see Therapeutic drug monitor
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u Unique value effect This term is
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Z Z variate A random variable which
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