Mediation, of the Statistical Variety

It’s harder than you think to learn about causal mechanisms

This week we ask the question: if there are several ways to generate a policy outcome, how important is each route? This is the mediation analysis, beloved of psychologists and communication scholars and, we shall see, rather more difficult than they hope.

One of the most important questions we can ask when trying to unpack the idea of one ‘route’ to an outcome being more important than another, is: what precisely is the causal effect of interest? Perhaps unexpectedly, the answers to this question require some interesting - or dismaying depending on your preferences - metaphysical considerations. For example, is the effect of a job finding program on labour force outcome more mediated by the increasing confidence or improving skills?

Assuming for a moment that these are independent, is the causal effect of the confidence ‘route’ the difference between your labour market outcome when you get treated, your confidence is raised, and your skills improved, and the counterfactual you whose skills are improved because you were treated, but whose confidence is not increased, exactly as if you were not treated? And if so, who is this person who is and is not affected by the treatment?

Here we are only trying to define the causal effects; things get harder as we try to estimate it.

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