9. The authors concluded that retiring early at 55 or 60 was not associated with better survival than retiring at 65 in a cohort of past employees of the petrochemical industry. Why do think they put the conclusion in this way, rather than concluding that early retirement was associated with worse survival?
We might have a healthy worker effect, where people in ill health retired early for that reason. Hence there is good reason to expect that early retirement would be associated with increased mortality which is not actually the result of retirement itself. Indeed, there was much greater mortality before age 65 in early retirers than later retirers, suggesting that this happened.
The authors were interested in whether early retirement increased survival. They could have done a legitimate one-tailed test here.
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