Consider the query: Does a binary treatment X have a causal effect on a response Y through a causal pathway that does not involve the intermediate variable M? This query is often rephrased as: Does X have a direct causal effect on Y not through M? Direct effects have been formally defined in three different ways: the controlled direct effects (CDE), the natural direct effects (i.e. pure and total direct effects- PDE and TDE), and the principal stratum direct effects (PSDE). In this issue of the journal, Hafeman and VanderWeele (H&V) 7 provide novel minimal or near minimal conditions for identification of the CDE, PDE and TDE but do not consider the PSDE. In this commentary, we review inference for direct effects and the results of H&V. We also review the close relationship between the direct effects literature and the literature on instrumental variables and Mendelian randomization. 1 Formal Definitions To proceed, we review the formal definitions of the three types of direct effects. We first consider a study with baseline covariates C, a dichotomous treatment X

The Centre for Market and Public Organisation (CMPO) is a leading research centre, combining expertise in economics, geography and law. Our objective is to study the intersection between the public and private sectors of the economy, and in particular to understand the right way to organise and deliver public services. The Centre aims to develop research, contribute to the public debate and inform policy-making. CMPO, now an ESRC Research Centre was established in 1998 with two large

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The Centre for Market and Public Organisation (CMPO) is a leading research centre, combining expertise in economics, geography and law. Our objective is to study the intersection between the public and private sectors of the economy, and in particular to understand the right way to organise and deliver public services. The Centre aims to develop research, contribute to the public debate and inform policy-making. CMPO, now an ESRC Research Centre was established in 1998 with two large

Childhood obesity has become a serious public health problem worldwide. The United States Centers for Disease Control and Prevention estimates that one in three children and adolescents are currently overweight/obese [1]. The adverse complications of childhood obesity on physical health and psychosocial development are tremendous [2]. Moreover, pediatric obesity may substantially increase future burden of cardiovascular disease and other morbidities in adulthood [3,4].