Dictionary of Evidence-based Medicine.pdf

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Dictionary of Evidence-based Medicine 35 The contingency table shown is a 3 row x 2 column table and is usually referred to simply as a 3 x 2 table; 2x2 tables are particularly commonly encountered. Multidimensional contingency tables are widely used in advanced categorical data analyses but require advanced graphical multivariate methods for representation on paper. Contingent valuation Estimates of willingness to pay for goods for which there is no market can be obtained using survey methods. Individuals are asked how much they would be willing to pay to avoid or overcome a certain state (e.g. avoidance of a particular morbidity). Using such an approach, sometimes referred to as 'contingent valuation' or expressed preference, the questions can be either open-ended or binary (Figure 5). In the former, the patient is asked the maximum amount he or she would be prepared to pay for the current treatment in a face-to-face or telephone interview or questionnaire. In order to improve response rate, response aids such as a bidding game are used. The patient is shown a value which is either rejected or accepted. The value is then altered until the maximum payment which is acceptable to the patient is identified. Alternatively, the patient may be shown a range of payment values from which a choice is made. The open-ended approach Figure 5 Contingent valuation: The calculation of willingness to pay using the binary contingent valuation method
36 Dictionary of Evidence-based Medicine is subject to starting point bias and because of this the binary contingent valuation method is becoming more popular. With this method, each respondent is asked to accept or reject a single bid or payment which will have to be paid in exchange for the treatment being evaluated. A patient who accepts the bid is assumed to have a maximum willingness to pay in excess of that amount. Conversely, one who rejects the bid is assumed to have a maximum willingness to pay which is lower. By stratifying a sample of respondents and presenting the same bid to each member of a given stratum, it is therefore possible to estimate the proportion of individuals who would be willing to pay each bid. A graph of bid value ($) against proportion of respondents willing to pay can then be drawn. The mean willingness to pay is obtained by estimating the area under the curve using appropriate statistical methods. Using these methods, it is possible to take account of variables such as income or educational level which may affect willingness to pay. The binary approach is sometimes referred to as a closed-ended method which is considered separately from the contingent valuation method (Mitchell RC, Carson RT (1989) Using surveys to value public goods: resources for the future. Washington DC. Johannsson P-O (1995) Evaluating health risks. Cambridge University Press, Cambridge). Continuous variable A variable which can take an infinite number of values (e.g. height and weight) within a range. In practice, statisticians often treat variables with more than about ten values as continuous. Contract curve (see under Efficiency) Convenience sample A convenience sample is made up of individuals or groups selected by the investigator because they were convenient at the time or location of sampling. Cost The cost of a product or service can be defined as the monetary value of the resources consumed in its production or delivery. Such costs can be subdivided into direct costs, which involve actual transfer of money, and indirect costs, which represent resources committed but unpaid. Some types of resources are required to be committed irrespective of level of
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Page 4 and 5: Preface This book was conceived at
Page 6 and 7: A Ability to pay Ability to pay is
Page 12 and 13: Figure I Audit: Clinical audit cycl
Page 14 and 15: B Balanced design In an experiment,
Page 66 and 67: Figure 9 Concept of externality Dic
Page 72 and 73: G Game theory Game theory, first de
Page 86 and 87: I ICD (see International Classifica
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Dictionary of Evidence-based Medici
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K Keynes, John Maynard (1883-1946)
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M Macroeconomics Macroeconomics is
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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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