2DkcTXceO

340 Lessons in biostatisticstime for the vinyl-chloride workers whose employment continued beyond thatpoint should have been assigned to the 0–14 group. When this correction wasmade, the 15+ year exposure group had a slightly higher mortality ratio thandid the 0–14 year group.Faculty at McGill University in Montréal, Canada, have repeatedly calledattention to erroneous conclusions in the medical literature stemming fromimmortal time bias. One recent article takes issue with the finding that actorswho won an Oscar lived on average nearly four years longer than those in amatched control group (Sylvestre et al., 2006). The authors pointed out that,as long ago as 1843, William Farr warned against the hazards of “classifyingpersons by their status at the end of follow-up and analyzing them as if theyhad been in these categories from the outset” (Farr, 1975). Farr continued“... certain professions, stations and ranks are only attained by personsadvanced in years; and some occupations are followed only in youth;hence it requires no great amount of sagacity to perceive that ‘the meanage at death’ [···] cannot be depended on in investigating the influenceof occupation, rank and profession upon health and longevity.”Noting the relatively early ages at death of Cornets, Curates and JuvenileBarristers, he concluded wryly: “It would be almost necessary to make themGenerals, Bishops and Judges — for the sake of their health.”Mistakes are made even when investigators are seemingly aware of theproblem. A 2004 report in The New England Journal of Medicine examinedthe effect on survival of a delay in kidney transplantation among children withend stage renal disease. The authors stated:“Delay in kidney transplantation as a potential risk factor for earlydeath was analyzed by comparing mortality among groups with differentlengths of time until transplantation. To account for survival bias,delay as a predictor of death was analyzed beginning 2 years after theinitiation of renal replacement therapy. There was no significant differencein mortality observed among groups with different lengths oftime until transplantation (Fig 3)” (McDonald and Craig, 2004).Close examination of their Figure 3, however, leads to a different conclusion.Survival curves from two years after onset of renal replacement therapy(dialysis or transplant) were shown separately for those with preemptive transplant(no delay), less than one-year delay and 1–2 years delay, categories basedon information available at the start of follow-up at two years. They are inthe anticipated order, with the survival outcomes best for those having hadan immediate transplant followed in succession by those having had a 0–1 or1–2 year delay. Had the authors simply added a fourth curve for those not yettransplanted by year 2, they would have found that it lay below the others.This would have confirmed the anticipated rank order in survival outcomesunder the hypothesis that longer delay increased subsequent mortality. However,they mistakenly split the fourth group into those who never received a