Theodore D Cosco
Longitudinal research methods have been a staple of gerontological research, providing myriad insights into the processes of ageing and the mechanisms that underpin the heterogeneity of later life. Although there is much to be learned from cross-sectional studies, i.e. studies that collect data at one point in time, longitudinal designs, i.e. studies that collect data at two or more points in time, are able to provide greater insights into change. They enable us to measure age-related changes in individuals and to help pinpoint the factors that are driving these changes. Without data collected over several waves, it may be very hard to reach firm conclusions about causation.
Suppose we conduct a cross-sectional study that demonstrates a relationship between depression and loneliness in older adults. What the study cannot tell us is whether participants are depressed because they are lonely or lonely because they are depressed. If, however, we examine this relationship over several waves of data collection in a longitudinal study we may be able to identify a temporal relationship between these variables, i.e. whether depression occurs before loneliness or vice versa. And this would provide stronger evidence as to the direction of causality. Longitudinal studies have helped to distinguish correlation and causation in the aetiology of health and disease.
There are many different types of longitudinal research, but they generally fall under three categories, i.e. trend, panel, and cohort studies, each collecting information from individuals at different time points (at least two). Trend studies take a cross-sectional snapshot of a population at several points in time. Census studies use a trend design, collecting data from a group of individuals that is not necessarily the same group at each data collection wave. Whilst these are useful in tracking overarching patterns, they do not provide any insight into change at the individual level. Panel studies are an improvement over trend designs in that they collect data from the same group of study participants for two or more waves of data collection. In this way researchers are able to model trajectories of change in individuals and to look at what sorts of factors are associated with these changes. An expansion of the panel design is the cohort study. This design follows a group of individuals that have a common factor that unites them, e.g. age, to examine change over time.
The UK is home to world-leading longitudinal birth cohort studies, i.e. cohort studies that follow a group of individuals born in the same year from birth onwards. The oldest of these studies is the National Survey on Health and Development started in 1946 (which has been previously covered in an OIPA blog), whose study participants celebrated their 70th birthdays in 2016. These studies have been followed by birth cohorts in 1958 (National Child Development Study), 1970 (1970 British Cohort Study), and 2000 (Millennium Cohort Study). The life-course perspective that these studies provide – by tracking individual lives from birth onwards - has greatly contributed to our understanding of how we age, and it is good to know that they are scheduled to continue to collect data and will continue to provide evidence on the determinants of individual ageing trajectories for the foreseeable future.
The study of ageing is a study of change over time, and to do it well we have to use methods that are capable of tracking and measuring change: longitudinal studies. The evidence derived from large-scale cohort studies helps us to disentangle complex relationships between variables and resolve uncertainties about the direction of causality. And when these studies, like the British Birth Cohort studies, follow individuals from the very beginning of their lives, they enable researchers to gain new insights into the ageing process. Improved insights lead to more effective action, and in this case the ultimate aim is to find out as much as we can about the means by which adults can continue to be healthy and happy in their later years.
About the Author
Theodore D Cosco joined the Oxford Institute of Population Ageing in 2016 as a Research Fellow. He holds a Canadian Institutes of Health Research Postdoctoral Fellowship to conduct a project entitled “Resilience and healthy ageing across the life course” in conjunction with the MRC Unit for Lifelong Health & Ageing.
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