Process Tracing (rival-explanation adjudication)

When one thing has happened and several stories compete to explain why, the instinct is to tally evidence: pile up what supports each story and pick the side with the bigger pile. That rewards whichever explanation attracted the most loosely-relevant chatter. Process tracing refuses the tally. It weighs each piece of within-case evidence by its diagnosticity - its power to eliminate or confirm an explanation - so that one decisive observation outranks any amount of weak, ambiguous support. The durable move is to make each rival explanation concrete as a causal mechanism chain (the step-by-step way that story would have produced this outcome, and the observable fingerprints each step would leave), state those expected fingerprints before weighing the evidence, then type each piece of evidence by certainty (if the explanation is true, must we see this?) and uniqueness (could the rivals also produce it?). A single failed hoop test eliminates a rival no matter how much straw-in-the-wind support it had. The output is a rival-explanation evidence ledger: the rivals, each one's mechanism chain, every evidence item typed per rival, the surviving explanation with its residual uncertainty, and the single most decisive observation still missing. It is explicitly not a cross-case generalization, not a consistency-scoring matrix, and not a manufactured winner when nothing available is diagnostic.

When to Use

• One outcome has occurred and there are genuinely rival stories about why it happened: an incident postmortem with three competing root-cause theories, a churn spike (a pricing change versus a competitor launch versus an onboarding regression), a lost deal, a metric anomaly, a contested past decision.
• Mechanism-level evidence is available or obtainable - logs, timestamps, documents, sequence of events, who knew what when - that could discriminate the rivals rather than merely decorate them.
• The argument has become a shouting match between narratives, and the useful reframing is "what would I expect to see if THIS story were true that the others would not produce?"

When NOT to Use

• Do not use it when there are no rivals on the table. With a single causal story there is nothing to discriminate. Descend the levels of that one story with think-iceberg-model, or decompose its coverage with think-issue-tree. Process tracing needs at least two genuine rival explanations.
• Do not use it for cross-case generalization. "Does X generally cause Y?" or "which combination of conditions produces success across our markets?" is comparative and configurational work over many cases. Process tracing's jurisdiction is one case, N equals one. (That cross-case space is QCA's territory, rejected in this library for fit.)
• Do not run it on an all-straw-in-the-wind evidence pool. When nothing available is diagnostic, running the ritual anyway produces false confidence. The honest output is "non-diagnostic - here is the observation that would discriminate," never a manufactured winner. This is the central wall.
• Do not let it degenerate into an evidence-by-hypothesis tally matrix. Scoring every item against every hypothesis for consistency and picking the least-inconsistent is Analysis of Competing Hypotheses, whose controlled record with professional analysts is null-to-negative (see evidence/dossier.md). If there is no single case and no mechanism chain - just generic multi-hypothesis scoring - decline rather than becoming that matrix under another name. The value lives in the mechanism chains and the per-item typing, not in a tally.
• Do not assign test types after seeing the evidence. Grading a found item as a "smoking gun" post hoc inflates its diagnosticity and invites motivated grading. The expected fingerprints for each rival must be stated before the evidence is weighed.

Instructions