Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
* selection of sample area segments (e.g., blocks within the city)
* selection of sample housing units within sample segments (e.g.,
residences within the blocks)
* selection of sample individuals within sample housing units
* selection of sample time points within the monitoring period
(Callahan et al., 1995).
The optimal degree of clustering depends on the variability of the
survey variables between and within the clusters and the costs of
fieldwork relative to sample collection and analysis costs. Although
details of this approach are beyond the scope of this chapter, it
should be recognized that cluster sampling introduces correlation
among the sample individuals that affects the validity of the survey
estimates. Thus, tradeoffs between increased sample size achieved
through clustering and loss of validity must be considered carefully.
Details of multistage and cluster sampling may be found in Hansen et
al. (1953), Kish (1965), Cochran (1977), Kalton (1983), Kollander
(1993) and Callahan et al. (1995).
One concern with survey studies is maintaining participation of
subjects who did not initially volunteer. Another issue, which is more
conceptual, is subject selection for the more complex sampling
strategies. In particular, stratification factors need to be carefully
chosen so that potential confounders can be determined and the
adjustments can be made from the resultant effects. Important
considerations for the design of population-based (e.g., national or
regional) exposure surveys, including response rates and confounders,
are discussed by Whitmore (1988) and Callahan et al. (1995).
3.4.3 Other sample types
Non-probability sample studies ("anecdotal studies") may consist
of selecting a sample based on the self-reporting of conditions, such
as complaint cases for "sick building" syndrome. Data collected in
this manner are potentially subject to biased reporting. It is
difficult to generalize results unless causal relationships are very
strong or unless there is little reason to believe that a confounder
or an unmeasured significant factor is relevant. In general, such
studies are used for description or exploration of a given situation.
In particular, they can be used to evaluate the variability of
outcomes and explore unknown situations for further explanatory study.
When choosing subjects, it is useful to focus on variability in the
expected outcome and also on the likelihood of completing the study.
It is also helpful to focus on a simple, preferably dichotomous,
hypothesis. Extensive validation will be necessary before accepting or
rejecting the hypothesis since the generalization of the results is
uncertain.
The advantages of targeted anecdotal studies are the inexpensive
and quick ways in which they aid in the design of future studies. For
example, when exploring protocols, determining stratification
variables, potential biases and confounders, and identifying the units
of analysis, the use of cooperative volunteers can simplify field
operations. The uncertainty of the results of these studies is due to
potential biases from the non-random and possibly non-representative
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