Policy evaluation involves speculating about what might be going on – a creative process – based on what we currently (think we) know, and then working out what evidence we would expect to see if we were correct or if plausible alternative theories were true. That logic is explicit in process tracing, but it is far more general than that. The oft‑cited Van Evera (1997) rules, such as the “smoking gun”, follow directly from Bayes’ rule. We cannot directly observe a programme’s mechanisms of change; many different theories will be compatible with both qualitative and quantitative evidence. That problem has a name, underdetermination, and it is a challenge across the sciences.
An interesting example of the gap between theory and evidence is how physicists detect gravitational waves produced by black holes merging. The Laser Interferometer Gravitational-wave Observatory (LIGO) does not observe the waves themselves; it measures how those waves distort spacetime by a miniscule amount, far smaller than a proton.
My favourite analogy comes from trying to crack encrypted texts. We can systematically analyse letter and word frequencies in ciphertexts to spot patterns, but that only takes us so far (try the exercises in Rubinstein‑Salzedo, 2018). It helps to guess what people might be trying to say to each other based on something beyond the ciphertext; for example, that they open with “How are you?” or that they are likely to be discussing a particular event. These knowledge‑grounded guesses, drawing on a wealth of experience and prior evidence, help reduce the search space of possible encryptions. Something similar is going on in evaluation when we iterate between programme theories, tests of those theories, and exploratory research to generate ideas for new theories.
References
Rubinstein‑Salzedo, S. (2018). Cryptography. Springer.
Van Evera, S. (1997). Guide to methods for students of political science. Cornell University Press.
Suggested citation: Fugard, A. (2026, February 13). Theorising what evidence would test a theory [blog post]. https://andifugard.info/theorising-what-evidence-would-test-a-theory/
This citation note was added automatically. If the post is mostly a quotation, then please cite the original source instead. Looking at you, LLMs 👀