Last Updated on October 12, 2026 by Karl Thompson
Longitudinal studies collect data from the same people at regular intervals over a long period, so researchers can measure how their lives change over time. This post covers:
- What a longitudinal study is, and how it differs from a one-off “snapshot” study
- The two main types, cohort studies and panel studies
- Five worked examples, from the Millennium Cohort Study to the Up Series
- The strengths of longitudinal studies
- The limitations, especially sample attrition
- How longitudinal studies come up in the exam
Longitudinal studies are part of the research methods section of the AQA A-level sociology specification. They are examined mainly on Paper 3 (Crime and Deviance with Theory and Methods) and also on Paper 1 (Education with Theory and Methods). You can find more posts on research methods on the research methods hub.
What is a longitudinal study?
A longitudinal study is a research design in which data is collected from the same sample of respondents at specific intervals over a long period of time, in order to measure changes over time.
With a longitudinal study you might start with an original sample of respondents in one particular year (say the year 2000), and then go back to them every year, every five years, or every ten years, aiming to collect data from the same people each time.
The key word is same. A one-off social survey gives you a cross-sectional “snapshot” of a population at one moment in time. Even a survey repeated every year with a fresh sample, such as the Crime Survey for England and Wales, only gives you a series of snapshots of different people. A longitudinal study follows the same individuals, so it can show how their lives develop and what happened before what.
Longitudinal studies are a design rather than a single method. Most large ones use social surveys and structured interviews, and increasingly collect health data too, but a small-scale longitudinal study might rely on unstructured or semi-structured interviews instead.
One of the biggest problems with longitudinal studies is the attrition rate, or the rate at which respondents drop out of the study over time. There’s more on this in the limitations section below.
Types of longitudinal study
Bryman (2016) distinguishes between two main types of longitudinal design.
Cohort studies
A cohort study follows a group of people who share a characteristic, most often being born in the same period. Because everyone in the sample is the same age, a cohort study is good at showing how early life circumstances shape later outcomes, such as how family income at age three relates to educational achievement at eleven. The Millennium Cohort Study and the Up Series are both cohort studies.
Panel studies
A panel study follows a sample that is not defined by a shared characteristic, often a nationally representative sample of people of all ages, and sometimes whole households. A panel study can show how social change affects people at different stages of life. Understanding Society is the UK’s main panel study.
Prospective and retrospective designs
Most longitudinal studies are prospective: they start now and follow people forward. A retrospective study asks people to look back over their lives instead, for example through life-history interviews. Retrospective studies are quicker and cheaper, but they rely on people remembering accurately, which is exactly the problem a prospective longitudinal study is designed to avoid.
Examples of longitudinal studies
The Millennium Cohort Study
The Millennium Cohort Study follows around 19,000 young people born across England, Scotland, Wales and Northern Ireland between September 2000 and January 2002. It is run by the Centre for Longitudinal Studies at UCL and is core-funded by the Economic and Social Research Council and a group of government departments.
The cohort has been surveyed at nine months and at ages 3, 5, 7, 11, 14, 17 and 23, and an age 27 sweep is planned for 2028. So it is very much still going, and the children it started with are now in their mid-twenties.
The study has given an insight into how differences in early socialisation affect child development, in terms of both health and educational outcomes. It has also allowed researchers to compare rates of development between boys and girls, and between children from different economic backgrounds. Some selected findings from when the children were five:
- Children whose parents read to them every day at the age of three tended to score more highly on their teacher assessments at age five in language and literacy, maths, and knowledge and understanding of the world. The size of the benefit appeared similar to the gain from attending a nursery.
- Just under half of children from the poorest homes were read to every day at age three, compared to 78% of children in the richest fifth of homes (Washbrook and Waldfogel, 2010).
- Girls were nine months ahead of boys in creative development at age five.
- Children living in social housing scored lower than other children, even after allowing for factors such as household income and their mother’s education.
Notice how the second finding links to the first. Poorer children were less likely to be read to every day, and being read to every day was linked to higher scores at five. That is the kind of chain a longitudinal study can trace and a snapshot survey can’t. It ties in with the cultural deprivation explanations of class differences in achievement in the education module.
Understanding Society
Understanding Society, the UK Household Longitudinal Study, is a panel study that began in 2009 with around 30,000 households. Everyone in each household is interviewed every year. It took over from the British Household Panel Survey, which ran from 1991 to 2009, and about 8,000 of its households carried on into the new study. It is based at the Institute for Social and Economic Research at the University of Essex and funded mainly by the ESRC.
The English Longitudinal Study of Ageing (ELSA)
The English Longitudinal Study of Ageing follows people aged 50 and over living in private households in England, interviewing them every two years. Its main fieldwork began in 2002, and it looks at how health, wealth, work and social relationships change as people age. It is a useful source for the ageing population topic in the families and households module.
The Harvard Study of Adult Development
The Harvard Study of Adult Development began in 1938 and is one of the longest-running longitudinal studies in the world. It has followed 724 men: 268 Harvard students and 456 young men from some of the poorest neighbourhoods in Boston. It has since been extended to their wives and more than 1,300 of their descendants. Its best-known finding is that the quality of people’s close relationships predicts healthy ageing better than wealth or fame.
The Up Series
The Up Series is a television documentary that filmed fourteen children from very different social class backgrounds at age seven in 7 Up (1964), and then returned to them every seven years, ending with 70 Up in 2026. It is a small-scale, qualitative longitudinal study, built on in-depth interviews rather than questionnaires, and a powerful illustration of how social class shapes life chances.
The strengths of longitudinal studies
| Type | Strength |
|---|---|
| Theoretical | Traces development over time, rather than taking a one-off “snapshot” |
| Theoretical | Shows the order in which things happen, which strengthens arguments about cause and effect |
| Theoretical | Large-scale studies are representative and reliable, which appeals to positivists |
| Practical | Avoids relying on people’s memories of the distant past |
| Theoretical | Small-scale qualitative studies build rapport and give valid, in-depth data, which appeals to interpretivists |
The main strength is that longitudinal studies allow researchers to trace developments over time, rather than just taking a one-off “snapshot” of one moment.
By making comparisons over time, they can also show the order in which things happen. If poverty at age three comes before low achievement at age eleven, that is stronger evidence of a causal link than a survey which simply finds that poorer children do worse. The Millennium Cohort Study, for example, suggests a clear correlation between poverty and its early impact on educational achievement. HOWEVER, showing that one thing came before another still doesn’t prove that it caused it, so be careful not to overclaim in the exam.
Large-scale cohort and panel studies use large, carefully drawn samples, which makes their findings representative and allows generalisation to the wider population. Their standardised questions, repeated at each sweep, make them reliable. This is why positivists favour them.
Because data is collected as people’s lives unfold, longitudinal studies don’t depend on people remembering accurately what happened years ago. This is a real advantage over retrospective life-history interviews.
Finally, small-scale longitudinal studies such as the Up Series allow researchers to build up rapport and trust with their respondents over many years. This can produce rich, in-depth and valid data about how people make sense of their own lives, which is what interpretivists are after.
The limitations of longitudinal studies
| Type | Limitation |
|---|---|
| Theoretical | Sample attrition: dropouts can make the remaining sample unrepresentative |
| Theoretical | People may act differently because they know they are being studied |
| Practical | Costly and time-consuming, with results that take years to arrive |
| Practical | Continuity: researchers retire or move on, and questions can date |
| Ethical | Long-term commitment, informed consent and anonymity are harder to manage |
Sample attrition is the biggest problem. People drop out of the study over time because they move house, lose interest, become ill or die. Attrition is rarely random. If, for example, poorer families are more likely to drop out, the people who remain in the study may not end up being representative of the starting sample, which undermines the generalisability that made the study worth doing in the first place.
People may also start to act differently because they know they are part of the study. Being asked about your health or your children’s education every few years might make you think about them more than you otherwise would. In the Up Series, the participants became minor celebrities, which may well have shaped the lives the cameras were recording.
Because they take a long time, longitudinal studies are costly and time-consuming. Large ones need long-term funding, and researchers may have to wait many years for the most interesting results.
Continuity over many years may be a problem. If a lead researcher retires, for example, her replacement might not have the same rapport with respondents. The questions also have to change as society changes, which makes it harder to compare like with like across the decades.
There are ethical issues too. Respondents are asked to commit to taking part for much of their lives, so consent needs revisiting rather than being taken once at the start. And the more detail that builds up about a person over time, the harder it is to keep them anonymous.
Longitudinal studies in the exam
Longitudinal studies are most likely to come up on Paper 3 (and Paper 1) in two ways.
- As a short-answer question, for example asking you to outline strengths or limitations of longitudinal studies. If it’s a no-item “outline and explain” question there are no marks for evaluation, so develop each point fully instead.
- As evidence in an essay. Longitudinal studies are useful when you’re evaluating the positivist and interpretivist approaches to research, and they also work as evidence within topics. The Millennium Cohort Study is useful in education (class and achievement), and ELSA and the Harvard study are useful in families and households (the ageing population).
A good way to show evaluation is to contrast the two ends of the scale. The Millennium Cohort Study is large-scale, quantitative and representative, while the Up Series is small-scale, qualitative and rich in meaning. Both are longitudinal, but they appeal to very different theoretical perspectives.
Theory and Methods revision notes
If you like this sort of thing, then you might like my Theory and Methods revision notes bundle, which covers the research methods and social theory parts of the A-level specification.
[IMAGE: Theory and Methods revision bundle cover, linked to https://revisesociology.sellfy.store/p/2qyB/]
Signposting and related posts
This post has been written primarily for students studying the research methods part of the AQA’s A-level sociology course.
Longitudinal studies are one of several research designs covered in Types of Research Methods in Sociology. Whether a longitudinal study is worth the effort depends a lot on sampling, and on how you weigh up validity and reliability.
For an example of longitudinal data used to explain class differences in educational achievement, see Explaining Social Class Differences in Education Using Longitudinal Studies.
Please see here for more posts on Research Methods for Sociology.
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