Analysis of Competing Hypotheses (ACH)¶

Why It Matters¶

Humans naturally gravitate toward the first plausible explanation and then cherry-pick confirming evidence. ACH reverses this: it identifies all reasonable hypotheses, lists all evidence, and systematically evaluates which evidence is consistent or inconsistent with each hypothesis. This reveals which hypotheses cannot be disproven - a more reliable basis for judgment than which hypotheses have some supporting evidence.

The Key Move¶

List all plausible hypotheses (including unlikely ones). List all significant evidence. Create a matrix where each cell answers: "Is this evidence consistent with this hypothesis?" Focus on inconsistencies - evidence that refutes hypotheses is more diagnostic than evidence that confirms. The hypothesis with the least inconsistent evidence (not the most consistent) is most likely. Explicitly state what evidence would disprove the leading hypothesis.

Classic Application¶

CIA analysis of Soviet missile capabilities during the Cold War. Rather than confirming suspected capabilities, analysts used ACH to identify what evidence would distinguish between "upgrading existing systems" vs "developing new weapons class" vs "strategic deception." The method revealed that most evidence was consistent with all hypotheses, forcing analysts to seek more diagnostic information.

Surprising Application¶

Medical differential diagnosis. Rather than confirming the most obvious diagnosis, physicians using ACH list all conditions consistent with symptoms, then systematically identify which symptoms rule out which conditions. This reduces misdiagnosis from premature closure. One emergency department reduced diagnostic errors by 23% after implementing ACH protocols.

Failure Modes¶

Treating absence of inconsistent evidence as proof (it may just mean you haven't looked). Listing too few hypotheses (excluding the correct one from the start). Making hypotheses too similar (they should be genuinely distinct). Treating all evidence as equally diagnostic (some evidence is consistent with everything). Using ACH when you need speed more than thoroughness (it's time-intensive).

Go Deeper¶

Heuer, "Psychology of Intelligence Analysis" Chapter 8. Richards J. Heuer Jr. invented ACH specifically for intelligence analysis. Schum, "The Evidential Foundations of Probabilistic Reasoning" for the formal epistemology.