6. Why did they adjust for socioeconomic group? What would we expect the effects of this adjustment to be?
Socioeconomic group is well known to be a predictor of mortality. People in higher socioeconomic groups have lower average mortality at any given age and longer average survival than those in lower socioeconomic groups. Also, age at retirement is related to socioeconomic group. The early retirers tended to be from higher socioeconomic groups than the later retirers. This could produce a difference in survival between early and late retirers which was due to differences in socioeconomic group rather than age at retirement itself. The effect would be to remove at least some of the spurious effect on survival of the differences in socioeconomic groups. We expect it to change the estimated hazard ratio for age at retirement.
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