Multivariate models were run. Dioxin exposure (variable
under scrutiny) and administrative departments (to
account for their heterogeneity) were forced into the mod-els. Then, the 5 remaining independent variables were
introduced as spline functions. To assess the smoothing
added value, we followed the ad hoc approach proposed
by Wood et al. [20]. If the estimated degrees of freedom
were close to 1 (and a linear function was therefore esti-mated), and if the generalized cross validation (GCV)
score was higher than the GCV for the unsmoothed
model, then the original unsmoothed variable was pre-ferred to the spline-smoothed version. Finally, using
Akaike's information criterion for covariate selection, a
backward stepwise selection was applied.