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METHODOLOGY
Peters TJ, Eachus JI. Achieving equal probability of
selection under various random sampling strategies. Paediatr Perinat
Epidemiol. 9(2), 219-24. 1995.
The underlying objective of epidemiological investigations
is to extrapolate results from a sample to the relevant population. The simplest
way of achieving this is to adopt a sampling strategy in which each individual
in the population has the same chance of being selected--that is, to employ an
'equal probability of selection method' (epsem). The easiest ways of achieving
this are to use simple random sampling or stratified random sampling with a
constant sampling fraction. These strategies are often impracticable, however,
particularly in large investigations covering a wide geographical area where
resource implications dictate a more complex approach such as multi-stage or
cluster sampling. Following detailed definitions and appropriate illustrations
of these terms, the main purpose of this paper is to provide a working guide of
how to achieve epsem using these various random sampling techniques. In brief,
for multi-stage sampling with the rare feature of equal-sized first stage units,
epsem is achieved by applying the above simple or stratified approaches to the
first stage units. Even in the more realistic scenario of unequal first stage
units, the same options apply provided that a fixed proportion of second stage
units are to be selected (cluster sampling is in fact just one example of this,
with 100% sampling of second stage units). If on the other hand a fixed number
of second stage units are to be selected then for epsem the first stage units
should be selected with each one having a probability proportionate to its size.
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