Design Gate — Pin the Identifying Assumption Before Anything Else

Skill type: DISCIPLINE-ENFORCING. This is the entry point of the social-science methods spine, not an analysis engine. It routes a study to the right design family and forces the one decision every downstream gate depends on: what identifying assumption licenses the causal claim? It does not run models — for execution it hands off to the analysis skills.

The Core Rule

A CAUSAL CLAIM IS ONLY AS GOOD AS ITS IDENTIFYING ASSUMPTION — NAME IT FIRST.

Design is chosen by the question and the data-generating process — who was selected, who was assigned, what varies and when — not by which method is fashionable or convenient. A quasi-experiment is an observational design that earns a causal interpretation only by committing, up front, to an assumption that makes the effect identified. State that assumption before touching an estimator; if you cannot defend one, the claim is associational, not causal.