The first thing study teams need to decide is who the study will focus on.
Think back to the three examples in the last section – each has a different sample population:
Most studies select their sample from within certain geographic limits. This might be for practical or scientific reasons. The geographic limits could be very small, for instance a city or county, or very large, such as the whole of the UK.
The first two examples are known as cohort studies and target specific groups or sections of the population. Cohort study samples share a common experience at a particular point in time. For example, a birth cohort follows children born within a specific period. Other cohorts follow groups of students in the same year at school, patients diagnosed with a certain disease at a particular point in time, or new recruits entering an organisation or industry in a given year.
Some studies, like Understanding Society, target the UK population as a whole. One challenge this presents is the fact that the population is always changing.
Studies that seek to represent the whole population must be ‘dynamic’ – that is, there needs to be a way in which new members can join the sample. Otherwise there is a risk that, over time, the sample will become increasingly different to the population it is meant to represent.
Understanding Society creates a dynamic sample by including people who move into participating households. For example, if the child of a participating household leaves home to move in with a partner, the partner will join the sample. Similarly, if a couple breaks up and forms two new households, both new households become part of the sample.
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