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Annexes 202educated subjects. This may imply that one cannot distinguish the association betweeneducation and the cognitive level from the association between education and tools used formeasuring cognition. A way to deal with these problems is to study the change over time of acommon latent cognitive factor underlying the various cognitive tests that are measuredrepeatedly. This common latent cognitive level, a potentially better measure of globalcognitive degradation with age, can be obtained by a nonlinear latent variable model forlongitudinal multivariate data in which psychometric tests are assumed to be nonlinearparameterized transformations of their common factor [22]. The common factor is a latentprocess representing the common cognitive process underlying the psychometric tests. It isrelated to age through a linear mixed model [16, 22]. By introducing covariates like gender oreducation both in association with the latent process and in association with eachpsychometric test, this model allows to compare the intensities of association of covariateswith the different psychometric tests adjusting on the latent cognitive level, in addition to theestimation of the global association with cognitive level. Thus the effect of the covariate onthe ability to perform the test can be separated from its effect on the latent cognitive levelitself. The present analysis consisted in applying such statistical modelling to the Paquidstudy, a French population-based cohort of elderly subjects who have been followed for 13years with repeated measures of cognition. The aim was to estimate the true impact of genderand educational level on cognitive degradation with age.4