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Dictionary of Evidence-based Medicine 147 Significance level When setting up a hypothesis test, we start off by formulating a null hypothesis (H 0 ). In frequentist or classic statistics, we postulate a probability model such that the difference in effect between two treatments would be normally distributed with mean zero and a variance which we would estimate from the sample drawn. We also postulate an alternative hypothesis (Hj), such that the difference in mean is not zero. This alternative hypothesis will be accepted if the null hypothesis is rejected based on a previously set level of error which we would be prepared to accept. This error level (Type 1 error), relating to rejecting the null hypothesis when it is in fact true, is the significance level of the test. Sometimes the probability of a particular observation under the probability model postulated under H 0 is also referred to as the significance level. Smith, Adam (1723-90) Adam Smith, a Scot, is widely regarded as the father of modern economics. His book, An Enquiry into the Nature and Causes of the Wealth of Nations, published in 1776, is looked upon by many economists as the most influential book on economics ever written. His main thesis was that if each person were free to pursue his or her own interests, then nature would guide the economy as if it was an 'invisible hand' and there was no need for government intervention. Progress, he argued, would result from new technology and the specialization of labour. His ideas were further developed by others to lead to the school of thought now popularly referred to as the classical school. Societal perspective In analysing the impact of a health care intervention, the perspective adopted will affect the outcome and it is generally accepted that a societal perspective, which takes account of the effect on the social welfare of everyone, is preferable. In such an analysis, all medical and non-medical costs and all personal indirect costs, irrespective of who pays, are included. Health prevention strategies which show up poorly when more restricted perspectives are used, often show up quite well when this societal perspective is adopted. Specificity In diagnostic testing, the specificity is the proportion of subjects without disease who test negative.
148 Dictionary of Evidence-based Medicine Specific rate Specific rate is the frequency of a particular event (death, adverse reaction, disease, etc.) in a particular subset of the population. Examples of commonly used specific rates are age-specific rates and cause-specific rates. For example, infant mortality which is expressed as an age-specific rate (e.g. 10 per 1000 population) is often used as an index of quality of health care in comparative studies. Matrices are often used to simultaneously represent age-and disease-specific death rates. Staging bias (see under Bias) Standard deviation The standard deviation is a measure of the spread of a distribution and is equal to the square root of the variance. If the distribution is Gaussian (or normal) then the mean ± 1.96 standard deviation is expected to cover 95% of the measurements. The standard deviation of a population is normally referred to by the letter sigma (o) and that of a sample by the letter (s). Standard error The standard error (SE) is the standard deviation of a statistical estimator such as the mean or a regression coefficient. The standard error of the mean (SEM) is, for example, a widely used statistic as it describes the precision associated with the estimate of the mean. The standard error is equal to the standard deviation of the individual measurements divided by the square root of the sample size. Standard gamble The standard gamble is a method for estimating the utility of a particular outcome from a particular perspective. Judges are asked to choose between life in a particular known health state and a gamble with a probability p of obtaining complete cure and a probability (1 - p) of death. The probability p is altered until the judges have no preference for either state. The probability p is then a measure of the preference or utility for the health state. Standard normal variate The standard normal variate, commonly referred to as Z, is a normally distributed variable with mean zero and variance 1 (see under Normal distribution).
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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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M Macroeconomics Macroeconomics is
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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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