Gamma-Gompertz Mortality

Gamma-Gompertz-Makeham was considered but the extrinsic mortality parameter was excluded from the fitted model. The fitted model is compatible with plateau-like oldest-old mortality but does not prove that a mortality plateau exists. The gamma-Gompertz distribution was selected because it can capture mortality deceleration or levelling-off at the oldest ages. A mortality plateau at extreme ages naturally leads to a gamma-Gompertz representation of oldest-old deaths. The main model fixed the aging-rate parameter b at 0.09 to keep lifespan distributions realistic. The main lifespan model used in the article is gamma-Gompertz.