Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
small town in southwestern Australia. The many potential populations
of interest which this sample might generalize include: all people
living in that town; people living in a small southwestern Australia
town; people living in southwestern Australia; people living in
Australia; people living in any small town; people in general. In this
case, the sample population is not likely to provide a representative
sample of the latter two populations.
The appropriateness of the generalization is determined by
considering if the sample is randomly selected in such a way as to be
representative of the larger population of interest (Whitmore, 1988).
This randomization is in terms of the distribution of the collected
data. For continuous outcomes, the percentages of key attributes, such
as demographic factors, should be similar between the sample and the
population. However, when this is not possible, owing to limited
funding for example, a descriptive study (described below) can provide
credible data, although the extent to which these can be generalized
is limited.
3.4 Types of study design
Once the population is defined, then the attention shifts to
sampling strategies; in particular, comprehensive samples, probability
samples, and other types of samples. A comprehensive sample includes
all members of the selected population. In a probability sample each
member has a known likelihood of being selected. Simple random
sampling is a special case where each member of the population has
an equal probability of being selected. Other types of study groups
are selected on the basis of other characteristics, such as
availability or convenience.
3.4.1 Comprehensive samples
Complete populations can be used to collect a full picture of the
process being studied, especially when the total population is
relatively small such as families in a neighbourhood. In these cases,
an exhaustive collection of measurements is taken from every potential
subject, and the completed data describe the situation exactly. There
is no sample variability except through the methods and procedures
used for measurement and monitoring. The main reasons for studies of
this nature are either a small population size, a need for a complete
evaluation of the problem, high potential risk, high variability among
units or legal requirements. The advantages of this type of study are
that a complete description of the exposure is given, and there is no
need for generalization because all potential subjects are covered.
The disadvantage of this approach, if the population is large, lies in
the expense: all individuals in all locations must be monitored at all
times.
3.4.2 Probability samples
Surveys consist of a random sampling of subjects from the
population of interest. This approach aims to remove selection bias
and is useful for generalizing results beyond the study sample. It is
important to distinguish that "random" does not translate to
"haphazard". A truly random sample is independent of human judgement.
Every unit in the total population has a known above-zero likelihood
of being included in the sample. Effective study design allows
researchers to draw statistically valid inferences about the general
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