A cohort is a redundancy, and a uniform rollout destroys it. Medicine learns without second chances because other bodies show the harm first (C-QZX3F4ETCT), but only while some of those bodies are exposed differently. A payment rule or protocol adopted everywhere at once leaves no unexposed comparison, so whatever harm it does merges with the background trend. Medicare's readmissions penalty (HRRP) took effect on one national date; readmissions fell, and whether deaths rose stayed contested, because both sides could only say their time series "does not imply any causal effect" or that mortality "could reflect secular trends". The clinical version: thrombus aspiration was "increasingly used" before a registry-based randomised trial found it "did not significantly reduce mortality"; the registry rebuilt the independent arm that adoption had removed. So P8's rule holds inside P4: a health system keeps its power to learn from harm only if it keeps independent units, by staggering or randomising rollout across sites instead of switching everywhere at once. That qualifies "make the safe choice the default": a default adopted everywhere at once is the one whose safety nobody can check afterwards. Diamond adds the second condition: independence is necessary, not sufficient. The Greenland Norse lived beside the Inuit for centuries and "would have had a better chance of surviving if they had learned from or traded with the Inuit, but they didn't." A comparison arm teaches only a system that is paid, and willing, to read it.

Evidence 4 passages

groundsSource document
Our use of an interrupted time series framework is effective in identifying discontinuities resulting from changes in health policy, but does not imply any causal effect of the HRRP on either readmissions or mortality.
groundsSource document
as an observational study, it cannot establish cause and effect among the HRRP implementation, readmissions reduction, and increased mortality risk. Because sociodemographic and care-of-transition factors strongly influence readmission risk, it is possible that interventions incentivized by the HRRP may have favorably influenced readmission trends, whereas trends in mortality could reflect secular trends for patients hospitalized with HF
groundsValue in Healthcare: Laying the Foundation for Health System Transformation, World Economic Forum, in collaboration with The Boston Consulting Group
to evaluate the effectiveness of coronary artery thrombus aspiration, a technique increasingly used along with percutaneous coronary intervention (PCI) for patients suffering from ST-segment elevation myocardial infarction (STEMI), a type of heart attack. The study found that routine thrombus aspiration before a PCI did not significantly reduce mortality and, therefore, did not contribute to healthcare value.
groundsSource document
The Greenland Vikings would have had a better chance of surviving if they had learned from or traded with the Inuit, but they didn't.

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