Longitudinal Studies are studies in which data is collected at specific intervals over a long period of time in order to measure changes over time. This post provides one example of a longitudinal study and explores some the strengths and limitations of this research method.
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. One of the biggest problems with Longitudinal Studies is the attrition rate, or the subject dropout rate over time.
The Millennium Cohort Study
One recent example of a Longitudinal study is the Millennium Cohort Study, which stretched from 2000 to 2011, with an initial sample of 19 000 children.
The study tracked children until the age of 11 and has provide an insight into how differences in early socialisation affect child development in terms of health and educational outcomes.
The study also allowed researchers to make comparisons in rates of development between children of different sexes and from different economic backgrounds.
Led by the Centre for Longitudinal Studies at the Institute of Education, it was funded by the Economic and Social Research Council and government departments. The results below come from between 2006 and 2007, when the children were aged five.
- The survey found that children whose parents read to them every day at the age of three were more likely to flourish in their first year in primary school, getting more than two months ahead not just in language and literacy but also in maths
- Children who were read to on a daily basis were 2.4 months ahead of those whose parents never read to them in maths, and 2.8 months ahead in communication, language and literacy.
- Girls were consistently outperforming boys at the age of five, when they were nine months ahead in creative development – activities like drama, singing and dancing, and 4.2 months ahead in literacy.
- Children from lower-income families with parents who were less highly educated were less advanced in their development at age five. Living in social housing put them 3.2 months behind in maths and 3.5 months behind in literacy.
The strengths of longitudinal studies
- They 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 identify causes. The Millennium Cohort study, for example suggests a clear correlation between poverty and its early impact on low educational achievement
The limitations of longitudinal studies
- Sample attrition – people dropping out of the study, and the people who remain in the study may not end up being representative of the starting sample.
- People may start to act differently because they know they are part of the study
- Because they take a long time, they are costly and time consuming.
- 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.