Process tracing

Process tracing is a case-based and theory-driven method for causal inference that applies specific types of tests to assess the strength of evidence for concluding that an intervention has contributed to changes that have been observed or measured.

This includes identifying and investigating alternative explanations for these changes.

Process tracing involves four types of causal tests:

Straw-in-the-wind test: These tests provide weak support for an explanation without definitively ruling it in or out, as the evidence could also fit other explanations. They can be used early in an analysis to identify areas that are worth investigating further.

Hoop test: These tests establish necessary criteria that a causal explanation must meet to remain viable. For example, a hoop test can involve temporal sequencing – a possible cause must have preceded the outcome of interest.

Failing a hoop test eliminates a possible causal explanation; passing a hoop test provides support for a possible causal explanation but does not prove it. The metaphor refers to passing through a hoop in order to remain in the contest.

Smoking gun test: These tests provide sufficient evidence to support a causal explanation. The metaphor refers to finding a murder suspect holding a recently fired gun.

Passing a smoking gun test provides strong support for a causal explanation and weakens alternative explanations without definitively ruling them out. Failing a smoking gun test does not eliminate a possible causal explanation but it does weaken it.

Doubly-definitive test: These provide strong evidence that both confirms a particular causal explanation and rules out other explanations.

In practice, evidence of this kind is rarely available for evaluations.



Examples and reflections on practice

Learning materials

Befani, B., D'Errico, S., Booker, F. & Giuliani, A. (2016). Clearing the fog: new tools for improving the credibility of impact claims.

INTRAC. (2017). Process Tracing.

Punton, M. & Welle, K. (2015). Straws-in-the-wind, Hoops and Smoking Guns: What can Process Tracing Offer to Impact Evaluation?. CDI Practice Paper 10, Brighton: IDS.

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