Section 1: What are longitudinal studies? From the CLOSER Learning Hub Module: Introduction to longitudinal studies

What is a longitudinal study? A longitudinal study is a prospective observational study that follows the same subjects repeatedly over a period of time. The UK is home to the largest and longest- running portfolio in the world. A longitudinal study is a prospective observational study that follows the same subjects repeatedly over a period of time. The UK is home to the largest and longest-running portfolio in the world. Definitions Observational studies focus on observing the characteristics of a particular sample without attempting to influence any aspects of the participants’ lives. They can be contrasted with experimental studies, which apply a specific ‘treatment’ to some participants in order to understand its effect. Prospective studies are a type of study that watches for outcomes, such as the development of a disease, during the study period and relates this to other factors, such as suspected risk or protection factor(s).

This slide depicts the longitudinal studies in the CLOSER partnership. The UK is most well-known for its birth cohort studies, which each follow a group of people born at a particular point in time throughout their entire lives. The UK is also home to Understanding Society, the largest household panel study of its kind anywhere in the world. Instead of following individuals, this study follows whole households of people through time. There are also other kinds of longitudinal studies, such as those following a group of people with a particular disease, or a cohort of students leaving university. In this module, we will mostly draw on examples from longitudinal birth cohorts and household panels.

What information do they collect? Longitudinal studies collect a broad range of information about different areas of their participants’ lives. This makes them incredibly valuable when looking at the way different aspects of our lives interact with each other. For a number of areas of life, it is important that longitudinal studies collect the same information every time they visit their participants. For instance, most studies will ask adult participants at each interview if they are employed, and if so, what sort of job they have and how much they are paid. Other information collected will relate to the participants’ stage of life. For instance, studies will ask questions about school while the participants are growing up, and about employment and retirement as they get older. The questions asked will also depend on the scientific purpose of the study. Some studies have a specific focus, and so concentrate on collecting very detailed information about the particular topic of interest. Others cover a wide array of topics in less detail.

Where does the data come from? Cohort study respondents: Study participants Parents Important people in the study members’ lives at particular ages (e.g. siblings, grandparents, teachers, partners, own children) Household panel respondents: All members of a household (whether they are a family or just sharing accommodation) In cohort studies, information is usually collected from the study participants directly (or from their parents when they are very young). However, some studies also collect data from other people, including siblings, grandparents and teachers. Of course, in a household panel, all members of the household participate in the study. This could include family members, but also housemates or flatmates in the case of shared accommodation. Some studies link individual survey responses to administrative (government) records, with the consent of the participant.

Data collection methods Questionnaires Cognitive assessments Clinical measurements Qualitative information Linked administrative (government) records Data collection methods can include: responses to survey questions about participants’ circumstances, behaviour and attitudes cognitive assessments, for example, exercises that test participants’ memory or reading ability clinical measurements such as height, weight and blood pressure that are carried out by trained personnel as part of a face-to-face interviews clinical samples, for example, blood, hair or saliva qualitative data from in-depth interviews or long-form answers to survey questions – for example the 1958 National Child Development Study carried out in-depth qualitative interviews with participants at age 50 linking study results to administrative data from government records (for example, a record of the participant’s exam results or hospital visits).