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06th Feb 2019

There has been interest in finding separate effects of age, year and birth cohort for decades, in both the biological and social sciences. However, the exact collinearity between these three (Age = Year – Birth Year) lead to difficulty in estimating these effects. Because of this, it is impossible to estimate near-linear effects (or linear components of these effects) without making strong assumptions about at least one of these. This is a problem for anyone interested in any of age, period and/or cohort patterns in a particular outcome.

There have been many attempts to ‘solve’ this identification problem without having to make strong assumptions – however in each case, it turns out such models are, in fact, making hidden assumptions that were not intended by the user, as I show with simulations. I then consider what researchers should do, drawing on literature from across the social, biological and health sciences. This includes consideration of non-linearities around linear APC effects (with both statistical and graphical techniques), strong and explicit assumptions based on theory (for example assuming there are no linear period effects), including constraints on certain parameters to estimate ranges within which other parameters must fall. I provide an example focusing on mortality in the twentieth century. In each case, these methods acknowledge that there is a ‘line of solutions’ of possible combinations of APC effects, and not a single answer that can be estimated empirically. None of these methods represent a solution to the identification problem – rather they are an honest acknowledgement of the problem, with an awareness that the methods are limited by their assumptions.

03rd Feb 2019

Objective: Attending childcare is related to greater childhood obesity risk, but there are few long-term follow-up studies. We aimed to examine the associations of childcare type, duration, and intensity with BMI trajectories from ages 10-42 years.

Research Design and Methods: The sample comprised 8234 individuals in the 1970 British Cohort Study, who had data on childcare attendance (no, yes), type (formal, informal), duration (4-5, 3-3.99, 0-2.99 years old when started), and intensity (1, 2, 3, 4-5 days/week) reported at age five years and 32563 BMI observations. Multilevel linear spline models were used to estimate the association of each exposure with the sample-average BMI trajectory, with adjustment for sex, father’s occupational class, and mother’s age of leaving full time education. A combined duration and intensity exposure was also examined.

Results: Childcare attendance and type were not strongly related to BMI trajectories. Results for the combined exposure revealed additive effects of childcare duration and intensity. Among participants who attended childcare 1-2 days a week, those who started when 3-3.99 years old had a 0.197 (-0.004, 0.399) kg/m2 higher BMI at age 10 years than those who started when 4-5 years old, and those who started when 0-2.99 years old had a 0.289 (0.049, 0.529) kg/m2 higher BMI. A similar dose-response pattern for intensity was observed when holding duration constant.

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