Dictionary of Evidence-based Medicine.pdf

Dictionary of Evidence-based Medicine 59
Excess risk
In a comparative study of risks associated with exposure to two therapies
or environments, the difference in risk can be expressed in absolute or
relative terms (see Risk difference, Relative risk and Risk ratio). The ratio
of the risk associated with one exposure divided by the risk associated
with the other is the relative risk. Suppose that the risk of developing
diarrhoea after receiving a course of antibiotic A is 5 in 100 and 2 in 100 for
antibiotic B, then the relative risk of developing this side-effect when
receiving A instead of B is 2.5 (i.e. 0.05/0.02). The excess risk is the difference
in risk expressed as a percentage or 150% in this case (i.e. 100 x (5-2)/2). A
common mistake in the literature is to interpret a risk ratio or relative risk
of 2.5 as a 250% or two and a half-fold increase in risk (Lam TH (1997)
Relative risks are inflated in published literature. BMJ. 315: 880).
Explanatory trial analysis (see under Intention to treat analysis)
Exponential survival model
The exponential survival model is characterized by the following survival
function, where T is a random variable representing failure time:
S(t) = P(T ^ t) = exp(-to)
The corresponding probability density function is given by/(y) = A exp(-Af).
The mean (j, of the exponential distribution is given by I/A and its variance
a 2 by I/A, 2 . The shape of the survival curve is of the type shown in
Figure 8.
Expressed preference method (see under Willingness to pay)
External validity
External validity refers to how applicable the results of a study are to the
target population. For example, a frequently expressed concern is whether
conclusions drawn from the results of clinical trials using patients adhering
to strict inclusion criteria are applicable to the population of patients
visiting primary care physicians. In other words, critics question the external
validity or generalizability of such clinical trial data to the average
primary care patient.