In mediation analysis, the effect of an exposure (or treatment) on an outcome variable is decomposed into two components: a direct effect, which pertains to an immediate influence of the exposure on the outcome, and an indirect effect, which the exposure exerts on the outcome through a third variable called mediator. Our motivating example concerns the relationship between maternal smoking (the exposure, X), birthweight (the mediator, M), and infant mortality (the outcome, Y), which has attracted the interest of epidemiologists and statisticians for many years. We introduce new causal estimands, named u-specific direct and indirect effects, which describe the direct and indirect effects of the exposure on the outcome at a specific quantile u of the mediator, 0 < u < 1. Under sequential ignorability we derive an interesting and novel decomposition of u-specific indirect effects. The components of this decomposition have a straightforward interpretation and can provide new insights into the complexity of the mechanisms underlying the indirect effect. We illustrate the proposed methods using data on infant mortality in the US population. We provide analytical evidence that supports the hypothesis that the risk of sudden infant death syndrome is not predicted by changes in the birthweight distribution.
Physical activity and inactivity are two independent dimensions over which children aggregate into distinct behavioural profiles. Read my new article ‘Probabilistic principal component analysis to identify profiles of physical activity behaviours in the presence of non-ignorable missing data’ in the Journal of the Royal Statistical Society: Series C at http://onlinelibrary.wiley.com/doi/10.1111/rssc.12105/abstract.
Read my new article ‘Improved transformation-based quantile regression’ in the Canadian Journal of Statistics at http://onlinelibrary.wiley.com/doi/10.1002/cjs.11240/abstract!
A typical summary statistic for temporal trends is the average percent change (APC). The APC is estimated by using a generalized linear model, usually under the assumption of linearity on the logarithmic scale. A serious limitation of least-squares type estimators is their sensitivity to outliers. We propose a robust and easy-to-compute measure of the temporal trend based on the median of the rates (median percent change – MPC), rather than their mean, under the hypothesis of constant relative change.
An exciting one-day meeting organized by the RSS General Applications Section will take place in London on 29th May 2013. In the morning, the workshop will have a tutorial in quantile regression with hands-on using the R package quantreg. The research session in the afternoon will have three excellent speakers. Full programme and registration details here.