title: Robustness of the BYM model in absence of spatial variation in the residuals.
abstract: BACKGROUND
  In the context of ecological studies, the Bayesian hierarchical Poisson model is of prime interest when studying the association between environmental exposure and rare diseases. However, adding spatially structured extra-variability in the model fitted to the data when such extra-variability does not exist conditionally on the covariates included in the model (over-fitting) may bias the estimation of the ecological association between covariates and relative risks toward the null. In order to investigate that possibility, a simulation study of the impact of introducing unnecessary residual spatial structure in the estimation model was conducted.
  
  
  RESULTS
  In the case where no underlying extra-variability from the Poisson process exists, the simulation results show that models accounting for structured and unstructured residuals do not underestimate the ecological association, unless covariates have a very strong autocorrelation structure, i.e., 0.98 at 100 km on a territory of diameter 1000 km."
authors: Inserm, U754, Villejuif, F-94807, France. aurelien.latouche@paris7.jussieu.fr
authors: Aurélien
authors: A
authors: Latouche
authors: Chantal
authors: C
authors: Guihenneuc-Jouyaux
authors: Claire
authors: C
authors: Girard
authors: Denis
authors: D
authors: Hémon
mesh_terms: D001499:Bayes Theorem; D003198:Computer Simulation; D004463:Ecology / Q000379:methods*; D008962:Models, Theoretical; D016012:Poisson Distribution