First, select your cohort

The first of the national cohort studies, the one begun one week in March 1946 has been the subject of a series of Radio 4 interviews and a newspaper article this week.  A cohort study follows a group of people (the cohort) over a period of time.  There are five big UK cohort studies begun in 1946, 1958, 1970, 2000 and 2012.  The three earlier studies were designed as medical projects, looking at maternal health and then at child development.  They have, however, made fascinating social history, and the questions posed have been so broad in topic that they are of interest to researchers in many fields. 

I am using the 1958 cohort, known as the National Child Development Study (NCDS), to look at volunteering in adolescence and young adulthood, and how those life experiences might influence adult behaviour.  As these cohort studies were designed with medicine in mind there are only a limited number of ‘social’ questions.  This study also has a rather large gap – no surveys took place in the 1980s.  I think there is a rather nice piece of meta-analysis to be done here: a paper on how long-running studies evolve over time, responding to changes in funding, fashions in research, and shifts in culture.  For example, no educationalist today would offer teachers the opportunity to tick a box to describe a pupil as “like a suspicious animal” – an option which was given to the teachers of the 1958 cohort when the children were eleven years old.

Unlike the 2000 and 2012 cohorts, who were selected from babies born throughout the year, the older cohort studies followed babies born in a particular week.  The 1958 cohort were all born in the week 3rd-9th March.  This selection method rules out a number of interesting avenues of research.  We can learn nothing about the health of people born in different seasons; nothing about the educational achievements of people born at the start or end of the school year; and we can’t control for these effects in other analyses either.  Survey methodology has changed in the last 40-odd years.  As part of my imaginary meta-analysis of cohort studies, I’d be interested to know whether there have been any advantages to the ‘one week in the year’ approach.  For example the ‘one week’ studies send birthday cards to all their participants, reminding them about the study and how important it is, urging them to stay in touch.  It’s part of the ‘story’ of the cohort.

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