When designing a longitudinal study, the
first thing that needs to be decided is who the study will focus on. Researchers might be interested in the population as a whole, or could select their sample of
participants based on factors such as age, location, or health. To select a suitable sample of participants, researchers need a list of everyone in
the population of interest. Examples of lists that have been used
include hospital, birth, and child benefit records. For surveys of the whole
population, studies can use a record of all addresses in the UK maintained by
the post office. Study teams usually select a sample that is larger than their ideal sample size to take into account the fact that response rates are never 100 percent. Another important consideration is attrition – that is
participants dropping out of the study over time. Some longitudinal studies
select a sub-sample of participants because the target population is larger
than the study needs.

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