title: Comparison of potential adjustment variables representing primary intellectual level in epidemiological studies on neurotoxicity.
abstract: BACKGROUND
  Primary intellectual abilities (PIA) are a confounder in epidemiological studies on neurotoxicity. A good measure of this confounder should be independent of age as PIA is an intrinsic ability. Furthermore, as PIA is related to health endpoints, any measure of PIA should reveal this association. This study is aimed at comparing vocabulary test, diploma and age at end of schooling properties as measures of PIA in a non-exposed population of workers.
  
  
  METHODS
  The design was a cross-sectional study of 413 non-exposed workers (203 women and 210 men) selected from a health check-up center. The effect of age on the vocabulary score was assessed using an analysis of covariance adjusted for diploma. Relationships between neuropsychological performances and vocabulary score, diploma and end of schooling age were, respectively, assessed using multiple linear regressions adjusted for age and gender.
  
  
  RESULTS
  Vocabulary score increased significantly with age, both for men and women. The increase was 0.14 word per year for women, and 0.18 word per year for men. The explained variance of the models evaluating the relationships between age at end of schooling, diploma, vocabulary test, and neuropsychological performances was quite similar for the three measures of PIA.
  
  
  CONCLUSIONS
  Vocabulary score was found to be age-related, even after adjustment for diploma. No difference was found between these three variables in terms of their relationship to neuropsychological endpoints. Moreover, the literature shows that vocabulary test performances are influenced by exposure to neurotoxic agents. These results suggest that vocabulary score could be of interest for participants of similar ages and similar diplomas. Otherwise, the other two variables would be better PIA measures in neurotoxicology studies.
authors.affiliation: Department of Epidemiology, French National Research and Safety Institute for the Prevention of Occupational Accidents and Diseases, Vandoeuvre, France. isabelle.rouch@chu-st-etienne.fr
authors.forename: Isabelle
authors.initials: I
authors.lastname: Rouch
authors.forename: Dominique
authors.initials: D
authors.lastname: Chouanière
authors.forename: Pascal
authors.initials: P
authors.lastname: Wild
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Fontana
authors.forename: Marcel-André
authors.initials: MA
authors.lastname: Boillat
mesh_terms: D000328:Adult; D003071:Cognition; D015986:Confounding Factors, Epidemiologic; D003430:Cross-Sectional Studies; D004781:Environmental Exposure / Q000009:adverse effects*; D005260:Female; D006306:Health Surveys; D006801:Humans; D007361:Intelligence Tests; D008297:Male; D020258:Neurotoxicity Syndromes / Q000453:epidemiology*; D009784:Occupational Diseases / Q000453:epidemiology*; D016272:Occupational Health; D014825:Vocabulary