1. Section 1 The beginning: aims, objectives and feasibility
From the CLOSER Learning Hub
Module: Study Design

2. Scientific aims and objectives
Aims and objectives dictate sample population and information collected
General vs specific scientific purposes
Common aim: to track change over time
The design of a longitudinal study is shaped by its overarching scientific purpose, and in some cases, the specific research questions that it seeks to answer. The study’s aims and objectives dictate who the participants will be and what information needs to be collected from them.
Some studies will have a very general scientific purpose, for example to determine how people’s experiences and circumstances in childhood affect the rest of their lives. Other studies will be designed for a much more specific purpose, for example to find out what factors influence whether young people go on to higher education, work or unemployment when they leave compulsory schooling.
Studies can also focus on certain aspects of life, for instance many studies are designed to look at health specifically, and the factors that lead to a healthy or unhealthy life.
But by their very nature, all longitudinal studies share one common aim – to track change over time.

3. Practical considerations
Respondent burden: how much effort and inconvenience will there be for participants?
Financial considerations: how will budget limit what can be achieved?
Legislation: are there any legal restrictions on what can be used as a sampling frame?
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 sample frames include the former UK Child Benefit Register, the Electoral Register, or the telephone directory.
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 add information about participants from government-held administrative data.

4. Prospective vs retrospective design
Prospective studies follow individuals over time and collect information as their characteristics and circumstances change
Retrospective studies start collecting data at a later point in people’s lives and fill in earlier ‘gaps’ by asking participants to recall information about their earlier lives, or by linking to administrative records
Perhaps the most fundamental design consideration for a new longitudinal study is whether it will take a prospective or a retrospective approach.
As described in the Introduction to longitudinal research module, prospective studies follow individuals over a period of time and collect information about them as their characteristics and circumstances change.
In contrast, retrospective studies start collecting data at a later point in people’s lives and fill in earlier ‘gaps’ by asking participants to recall information about their earlier lives, or by linking their survey responses to government-held administrative data (for example, tax records or scores on standardised school tests).
In reality, many studies use a mixture of both approaches. As no participants are constantly monitored, they will always be asked to remember back over the past year or few years, depending on when they were last interviewed. Longitudinal study teams make careful decisions about what information can be recalled with reasonable accuracy, and what information must be collected prospectively or ‘in the moment’.