Dictionary of Evidence-based Medicine.pdf

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Dictionary of Evidence-based Medicine 143 Risk difference (see under Risk) Risk ratio (see under Risk) Risk reduction In assessing and attributing risk, it is best to express the risk relative to a control (see under Risk). For example, in estimating by how much an antibiotic reduces the risk of traveller's diarrhoea, we could observe 100 patients receiving it and report how many (say five) develop diarrhoea. The estimate of absolute risk of diarrhoea would be 5/100 or 0.05. This figure, however, does not tell us by how much the antibiotic has reduced the incidence of diarrhoea. To do this, we may study a control group of patients given a placebo to obtain another estimate of risk of diarrhoea for this group of patients (say 10/100 or 0.1). The difference in the two rates (0.05) gives us an estimate of the risk reduction in absolute terms. For this reason this value is called the absolute risk reduction (ARR). On average, five fewer patients out of 100 treated would develop diarrhoea than if they received a placebo. The reciprocal of the risk reduction gives us the number needed to treat (see NNT), 20 in this instance. The NNT tells us that on average 20 patients need to be given antibiotics for one more patient to be protected from diarrhoea when compared to placebo. The risk reduction may also be expressed as a proportion or percentage of the risk in the control group to produce a relative risk reduction (RRR), expressed as a proportion or percentage. In the above example, the relative risk reduction is given by (0.1 - 0.05)/0.10 or 0.5. Expressed as a percentage the RRR is 50%. This appears more impressive than the ARR of 0.05 or 5%. It is worth noting that, perhaps confusingly, both the absolute and the relative risk reduction terms are relative to a control group. The terms used refer to how the values are expressed. The RRR is as a proportion or percentage of the control rate or risk while the ARR is on its own. Root mean square error The root mean square error (RMSE) is an estimate of the standard deviation associated with experimental error. For a sample of n observations x v x x ,...,x n , the root mean square error or standard deviation is defined as:
144 Dictionary of Evidence-based Medicine Rosner and Watts matrix This is a disability-distress matrix (see below) developed by Rosner and Watts giving weights or utilities for quality-adjusted life-years gained as a result of a particular intervention (Rosner). Table 8 The Rosner and Watts disability-distress matrix Disability None Distress Mild Moderate Severe None Slight Severe Severely limited performance No paid employment or education Chair-bound Bedridden Unconscious 1.000 0.990 0.980 0.964 0.946 0.875 0.677 -1.028 0.995 0.986 0.972 0.956 0.935 0.845 0.564 — 0.990 0.973 0.956 0.942 0.900 0.680 - — 0.967 0.932 0.912 0.870 0.700 0.000 -1.486 —
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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
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B Balanced design In an experiment,
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Figure 9 Concept of externality Dic
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G Game theory Game theory, first de
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I ICD (see International Classifica
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K Keynes, John Maynard (1883-1946)
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Page 100 and 101: M Macroeconomics Macroeconomics is
Page 112 and 113: N N of I trial An N of 1 trial is a
Page 116 and 117: o OBM (see Opinion-based medicine)
Page 140 and 141: Figure 25 Measuring quality of life
Page 150 and 151: S Scarcity Scarcity is often referr
Page 158 and 159: T TDM (see Therapeutic drug monitor
Page 162 and 163: u Unique value effect This term is
Page 170: Z Z variate A random variable which
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