Dictionary of Evidence-based Medicine.pdf

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Dictionary of Evidence-based Medicine 85 Figure 12 The concept of indifference Intention to treat analysis This is a method of analysis for randomized controlled trials whereby all patients randomly allocated to one of the treatments in a trial are analysed together irrespective of whether or not they completed or received that treatment. This is recommended to ensure that the benefits of randomization are not lost through introduction of biased selection at the analysis stage. Intention to treat analysis is also referred to as pragmatic trial or programme effectiveness analysis. This is to be contrasted with (i) efficacy analysis which compares outcomes only for subjects who have completed treatment with that intended at randomization (such an analysis is also referred to as an exploratory trial analysis or biological efficacy test analysis) and (ii) treatment received analysis which compares subjects according to which treatment they actually received irrespective of which treatment they were assigned to at randomization. This latter analysis is also termed 'as treated analysis' by some trialists and less kindly as 'garbage analysis' by other commentators (Newell DJ (1992) Intention to
86 Dictionary of Evidence-based Medicine treat analysis: implications for quantitative and qualitative research. International Journal of Epidemiology. 21: 837-41. Peduzzi P, Detre K, Wittes }, Holford T (1991) Intent to treat and the problem of cross-overs. An example from the Veterans Administration coronary bypass surgery study. Journal of Thoracic and Cardiovascular Surgery. 101: 481-7). While intention to treat analysis is rather straightforward to apply when the outcome is discrete, with continuous outcomes the analysis requires imputation of data and how to deal with those who actually switched treatments is more problematic. Interim analysis When designing a clinical trial, there are often areas surrounded by much uncertainty. For example, in calculating the sample size of the trial, we require an estimate of the variance of the patients' responses and there may not be much prior knowledge about this and a rough estimate has to be made. In other situations, for ethical reasons, we may wish to terminate a trial early if a treatment appears to be obviously superior to another from the early data. To enable us to decide when to stop or whether recruitment has to be increased, we need to do an analysis which may not be the final planned analysis. This is termed an interim analysis. As such, it is a scientific inferential process which uses appropriate statistical methods and hence has to be planned for with decision rules defined prospectively at the trial design stage. Such interim analyses are usually conducted when there is clearly a lack of intended effect, emergence of serious unpredicted adverse effects or overwhelming efficacy results in life-threatening or severely debilitating illness. Unplanned interim analyses outside these situations should not be conducted as they interfere with the internal validity of the trial (Sankoh AJ (1995) Interim analyses: an FDA reviewer's experience and perspective. Drug Information Journal. 29: 729-37). Internal validity Internal validity refers to whether conclusions drawn with respect to the specific population under study are valid. For example, in an observational study evaluating the significance of various potential risk factors for a particular disease, systematic differences in the characteristics of different groups pose a serious threat to its internal validity. In other words, an association which is identified between a risk factor and the disease may arise solely because the risk factor is associated with another, more significant one.
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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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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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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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