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.

Resources

Guides

  • Understanding process tracing

    This 2011 paper, from David Collier, outlines a new framework for process tracing to achieve greater systemisation of qualitative methods. This version includes some reflections in 2019 on subsequent developments.

  • Straws-in-the-wind, Hoops and Smoking Guns: What can Process Tracing Offer to Impact Evaluation?

    This 2015 paper by Melanie Punton and Katharina Welle explains the methodological and theoretical foundations of process tracing and discusses its potential application in international development impact evaluations, including two examples of its application.

  • Process tracing (PDF)

    This 5-page paper by INTRAC provides an overview of process tracing with reference to its use in evaluations of community service organisations (CSOs).

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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