{"id":"CPY-019","name":"Test one causal variable at a time when possible","domain":"copywriting","layer":"content_system","strength":"heuristic","human_logic":"IF an experiment must identify what caused a result, THEN change one primary variable or use a designed multivariate test with adequate traffic ELSE treat results as directional.","when":{"path":"experiment.goal","operator":"eq","value":"causal_attribution"},"then":[{"action":"design","target":"experiment","options":["single_primary_variable","powered_multivariate"]},{"action":"qualify","target":"experiment.result","when":"confounded","value":"directional_only"}],"else":[{"action":"retain","target":"current_text","reason":"No triggering condition is present."}],"unless":[],"because":"Confounded variants cannot reveal which change caused the outcome.","diagnostics":[],"examples":{},"source_ids":["BK-033"],"laka":{"smallest_sufficient_intervention":true,"primary_axes":["scope","depth","magnitude","reversibility"]},"requires_facts":["experiment.goal"],"tests":[],"sources":[{"id":"BK-033","title":"Scientific Advertising","creator":"Claude C. Hopkins","domain":"advertising","type":"Book","note":"Testing, measurement, offers, and accountable advertising.","url":null}],"defined_in":{"file":"16-copywriting-rules.json","title":"Persuasive and Direct-Response Copy Rules","slug":"copywriting-rules"}}