Tsai et al. (2005) studied the age at retirement and long term survival of an industrial population: using a prospective cohort study. They wanted to know whether early retirement was associated with better survival. They noted that
“Some researchers had concluded that early retirement harms health, attributing this to illness before retirement or the change of life events associated with retirement. On the other hand, there is a widespread perception that early retirement is associated with longer life expectancy and that retiring later leads to early death. The possible health benefits of retirement, such as reduced role demand and a more relaxed lifestyle, have been postulated to improve longevity among people who retire early.”
The sample were past employees of Shell Oil who retired at ages 55, 60, and 65 between 1 January 1973 and 31 December 2003. They looked only at those who reached the age of 65 and they compared survival beyond that point.
The following figure shows the survival after age 65 of subjects who retired early at age 55 compared with those who retired age at 65:
The authors say that there was a consistently lower probability of survival for employees
who retired at 55 (173 deaths) than for those who retired at 65 (462 deaths).
This difference was not, however, statistically significant (P = 0.09, log rank test).
Women accounted for about 11% (10% among early retirees and 12% among those retiring at 65)
of the study population during follow-up.
More than half of early retirees who reached 65
(57% who retired at 55 and 53% who retired at 60) were in the high socioeconomic group
whereas less than half (44%) of those who retired at 65 were in this group
The authors used the Cox proportional hazards model to estimate the hazard ratios
of death between the early and normal retirement groups, with adjustment for sex,
calendar year of entry into the study, and socioeconomic group.
After adjustment, employees who retired at 55 and were still alive at
65 had significantly higher mortality than those who retired at 65
(hazard ratio 1.37, 95% CI 1.09 to 1.73).
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.
Mortality was higher in employees who retired at 55 than in those who continued working.
1. What kind of graph is this?
What assumptions are required about the data?
Do you think they are plausible?
2. Why do you think the curves become steadily steeper as survival increases?
3. What is a log rank test and what can we conclude from this one?
4. What is the Cox proportional hazards model, what are its assumptions,
and why was it used here?
5. Why do they say that employees who retired at 55 and were still alive at 65
had significantly higher mortality than those who retired at 65, when no P value is given?
6. Why did they adjust for socioeconomic group?
What would we expect the effects of this adjustment to be?
7. Why did they adjust for sex, which is very similar in the
early and late retirement groups?
What would we expect the effect of this to be?
8. Why did they adjust for calendar year at entry, i.e. the year at which subjects
became 65 years old?
Tsai SP, Wendt JK, Donnelly RP, de Jong G, Ahmed FS. (2005)
Age at retirement and long term survival of an industrial population:
prospective cohort study.
British Medical Journal 331, 995.
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Last updated: 29 July, 2009.