The Oddity of Heterogeneity: A Blessing in Disguise

Damage accumulation is widely accepted as the central dogma of ageing, and it has been a long-standing belief that tobacco smokers must experience a faster rate of ageing than non-smokers. It is therefore puzzling as to why proportional hazard model is a popular choice in longitudinal studies given that its assumption assumes a constant hazard with increasing time. If the rate of ageing is accelerated, the hazard gradient of smokers d(log(μ(x)))/dx obtained from frailty parametric fit has to be steeper than non-smokers. This study examines the relative derivative for mortality d(log(μ(x)))/dx of British doctors born 1900–1909, and obtained estimates indicate that the rate of ageing is similar between smokers and non-smokers. A brief theorem is also elaborated to present the difference in life-years gained from interventions and policies by life-detrimental risk exposure; e.g. smokers 0.8; non-smokers 5.3 mins/day. The controversial assumption made in the central dogma of ageing, heterogeneity axiom and the application of proportional hazard models are unveiled in this condensed parametric analyses.