Taking birth year into account when analysing effects of maternal age on child health and other outcomes: The value of a multilevel-multiprocess model compared to a sibling model

Abstract

BACKGROUND When analysing effects of maternal age on child outcomes, many researchers estimate sibling models to control for unobserved factors shared between siblings. Some have included birth year in these models, as it is linked to maternal age and may also have independent effects. However, this creates a linear-dependence problem. OBJECTIVE One aim is to illustrate how misleading the results may actually be when attempts are made to separate effects of maternal age and birth year in a sibling analysis. Another goal is to present and discuss the multilevel-multiprocess model as an alternative. METHODS Infant mortality was chosen as the outcome. Births and infant deaths were simulated from a multilevel-multiprocess model that included two equations for fertility, with a joint random effect, and one equation for infant mortality, with another random effect. The two random effects were set to be correlated. The effects of the independent variables were taken from simpler models estimated from register data. Various sibling models and multilevel-multiprocess models were estimated from these simulated births and deaths. In some simulations, two standard assumptions about the random effects were intentionally violated. CONTRIBUTIONS The paper illustrates how problematic it is to include both maternal age and birth year in a sibling model. Also, if only maternal age is included, but along with other reproductive variables, small categories should be used. It is argued that a multilevelmultiprocess model may be used instead to separate effects of maternal age and birth year, but this approach also has limitations, which are discussed.

Maternal age and the risk of low birthweight and pre-term delivery: a pan-Nordic comparison

“…In a sibling comparison model, maternal age at birth and year of birth are perfectly collinear, meaning that the effect of maternal age cannot be identified. 15,33 This is illustrated in Model 2 in this study.…”

Birth Order Differences in Education Originate in Post-Natal Environments

“…We use family-level fixed effects when estimating the bivariate association between birth order and these outcomes, meaning we are comparing siblings within the same family. We control for sex and maternal age 33 , and run the models separately by sibship size. Consistent with other studies, we find that firstborn siblings have better educational outcomes than their later-born siblings.…”

Response to letter: Sibling models—An underused tool with limitations

“…It arises, for example, when effects of maternal age on a certain child outcome are estimated, and period indicators are added in an attempt to control for the secular trend in this outcome. 4 Furthermore, estimates of effects of reproductive factors on child outcomes may in some cases be biased because of so-called 'carryover'. One example of this occurs when the outcome is child death or something else with large impact on subsequent fertility, because the outcome for one sibling then affects the exposure (reproductive factors) for the younger sibling.…”