London School of Hygiene & Tropical Medicine

For predicting intervention outcomes in a new setting you need a context-local causal model of what is expected to happen there. A theory of change (ToC) for the intervention is a starting point. ToCs are ‘middle-level theories’: they aim for some, but not universal, general applicability. These are typically ‘arrows-and-variables’ models depicting what steps should occur in sequence but not the interactive factors necessary at each step, nor possible interrupters/defeaters.  For policy prediction these theories need context-local thickening. This requires an understanding of how each step produces the next, which in turn calls for middle-level theory of a different kind:  the middle-level principles (mechanisms) that govern that production. The context-local model allows better prediction of whether an intervention can work there, what it would take for it to do, what side effects might be and whether all this is affordable and acceptable in the context.

Nancy Cartwright, FAcSS is Professor of Philosophy at Durham University and a Distinguished Professor at the University of California, San Diego (UCSD). In the first half of her career at Stanford University she specialised in the philosophy of the natural sciences, especially physics; in the second half, at the London School of Economics and now Durham and UCSD, she has specialised in philosophy and methodology of the social sciences with special attention to economics. Her current research focuses on objectivity and evidence, especially for evidence-based policy.

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