Not all decisions about longitudinal study design are made for strictly scientific reasons. Some decisions are made based on what is practically feasible.
Respondent burden is a major preoccupation for longitudinal study teams. Many study participants lead busy lives and their decisions on whether to take part in a study will be influenced by how much effort and inconvenience it will be. Longitudinal study teams take into account how burdensome the study will be for participants when deciding the frequency and length of interviews, and what the participants are asked to do.
Financial considerations inevitably limit the scope of a study. Longitudinal studies can be very expensive to run. Costs may dictate the sample size, the frequency and length of interviews, and even the way the interviews are carried out (for example, in person, by phone or online).
Legislation can have an impact on study design. It can constrain what can be used as a sample frame (the list from which participants are selected), and how they can be contacted. Examples of current and past sample frames include the former UK Child Benefit Register, the Electoral Register and the Postcode Address File (a list of residential addresses in the UK).
The law dictates that some sample frames can only be used on an ‘opt in’ basis. This means that all eligible people in the sample frame must be contacted and asked whether they are happy for researchers to get in touch with them about taking part in the study. Only those who confirm they are happy for this to happen can then be contacted and asked to join the study.
Opting in produces a much smaller, and more biased, sample than opting out. In ‘opt out’ studies, participants are notified that they are in the study, and given the opportunity to withdraw if they wish. The longitudinal study team is given permission to contact all those people who have not opted out.
Legislation can also affect how straightforward it is to link information about participants from government-held administrative data. We will learn more about administrative data in the study content section.
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