Philosophical Studies

, Volume 147, Issue 1, pp 59-70

First online:

Open Access This content is freely available online to anyone, anywhere at any time.

What are randomised controlled trials good for?

  • Nancy CartwrightAffiliated withCentre for the Philosophy of Natural and Social Sciences, London School of EconomicsDepartment of Philosophy, University of California, San Diego Email author 


Randomized controlled trials (RCTs) are widely taken as the gold standard for establishing causal conclusions. Ideally conducted they ensure that the treatment ‘causes’ the outcome—in the experiment. But where else? This is the venerable question of external validity. I point out that the question comes in two importantly different forms: Is the specific causal conclusion warranted by the experiment true in a target situation? What will be the result of implementing the treatment there? This paper explains how the probabilistic theory of causality implies that RCTs can establish causal conclusions and thereby provides an account of what exactly that causal conclusion is. Clarifying the exact form of the conclusion shows just what is necessary for it to hold in a new setting and also how much more is needed to see what the actual outcome would be there were the treatment implemented.


Randomized controlled trials (RCTs) External validity Probabilistic theory of causality Causal inference Capacities Contributions