Dictionary of Evidence-based Medicine.pdf

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Dictionary of Evidence-based Medicine 121 Phase I studies (see Drug development) Phase II studies (see Drug development) Phase III studies (see Drug development) Phase IV studies (see Drug development) PK-PD modelling (see Pharmacokinetic-pharmacodynamic modelling) Point prevalence (see under Incidence) Poisson distribution The random variable X representing the number of events (x = 0, 1,..) occurring by time t in a Poisson process with mean rate X has a Poisson distribution with parameter Id as described in the following equation: The Poisson distribution can be used to model distribution of events not only in time but also in space. It has been used to model the occurrence of adverse drug reactions. The mean and variance of a Poisson distribution are both given by |a = \t. The Poisson distribution is an approximation to the binomial distribution B(n, *pi ) where n is the number of trials and ^ is the probability of success in each trial. Poisson process A Poisson process is one in which events occur spontaneously, at random, in time. Many phenomena of biological or medical interest can be modelled using the Poisson process (e.g. number of accidents per day). A Poisson process is specified by three postulates: (i) the probability of an event occurring during any small interval (t,t + St) is equal to (8t) \8t + o(8t)
122 Dictionary of Evidence-based Medicine where A, is the mean rate, t is the time and o(St) is a value which tends to zero as the small time interval tends to zero; (ii) the probability of more than one event occurring during a small interval is equal to o(St); (iii) the occurrence of events after any time t is independent of the occurrence of events before time t. The Poisson process described above is referred to as being stationary because of the assumed constant mean rate. The process can be extended to model more complicated processes such as by using mean rates which vary with time (non-homogeneous Poisson process). Population at risk The population at risk denotes the denominator in the calculation of event (e.g. death, disease or adverse reaction) rates in epidemiological studies. For example, when calculating the death rate for a particular community, the population at risk is usually taken as the estimate of the population of that community thought to be alive on July 1 of the calendar year concerned. In estimating the incidence rate of an adverse drug reaction, the population at risk is the number of patients exposed to the drug and observed for the adverse reaction. PORT (see Patient outcomes research team) Positive economics Positive economics is an approach to economic analysis which is only concerned with the consequences of different changes or policies and does not make judgements about the desirability of alternative allocation of resources. This is in contrast to normative economics which is concerned with analysing the desirability of different changes and policies. Normative economics is also referred to as 'welfare economies'. The normative approach is used when criteria are required to rank different options (Johansson PO (1991) An introduction to modern welfare economics. Cambridge University Press, Cambridge). Positive rate of time preference (see under Discounting) Post-randomization consent design In a conventional randomized controlled trial (RCT), informed consent is sought from patients to participate in the trial. Those who agree to
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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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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
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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