title: A graphical method to assess distribution assumption in group-based trajectory models.
abstract: Group-based trajectory models had a rapid development in the analysis of longitudinal data in clinical research. In these models, the assumption of homoscedasticity of the residuals is frequently made but this assumption is not always met. We developed here an easy-to-perform graphical method to assess the assumption of homoscedasticity of the residuals to apply especially in group-based trajectory models. The method is based on drawing an envelope to visualize the local dispersion of the residuals around each typical trajectory. Its efficiency is demonstrated using data on CD4 lymphocyte counts in patients with human immunodeficiency virus put on antiretroviral therapy. Four distinct distributions that take into account increasing parts of the variability of the observed data are presented. Significant differences in group structures and trajectory patterns were found according to the chosen distribution. These differences might have large impacts on the final trajectories and their characteristics; thus on potential medical decisions. With a single glance, the graphical criteria allow the choice of the distribution that best capture data variability and help dealing with a potential heteroscedasticity problem.
authors: Hospices Civils de Lyon, Service de Biostatistique, Lyon, France; Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France CNRS, UMR5558, Laboratoire de Biométrie et Biologie Evolutive, Equipe Biostatistique-Santé, Villeurbanne, France.
authors: Mad-Hélénie
authors: MH
authors: Elsensohn
authors: Hospices Civils de Lyon, Service de Biostatistique, Lyon, France; Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France CNRS, UMR5558, Laboratoire de Biométrie et Biologie Evolutive, Equipe Biostatistique-Santé, Villeurbanne, France.
authors: Amna
authors: A
authors: Klich
authors: Hospices Civils de Lyon, Service de Biostatistique, Lyon, France; Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France CNRS, UMR5558, Laboratoire de Biométrie et Biologie Evolutive, Equipe Biostatistique-Santé, Villeurbanne, France rene.ecochard@chu-lyon.fr.
authors: René
authors: R
authors: Ecochard
authors: Epicentre, F-75011 Paris, France.
authors: Mathieu
authors: M
authors: Bastard
authors: UMR U1027, INSERM, Université Paul Sabatier, Toulouse III; CeRSME (EA 2931), UFR STAPS, Université de Paris Ouest-Nanterre-La Défense.
authors: Christophe
authors: C
authors: Genolini
authors: UMI 233 "TransVIHMI" Institut de Recherche pour le Développement, Université Montpellier 1, F-34394 Montpellier, France.
authors: Jean-François
authors: JF
authors: Etard
authors: Hospices Civils de Lyon, Service de Biostatistique, Lyon, France; Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France Département de santé publique, Institut des Sciences Pharmaceutiques et Biologiques (ISPB), Université de Lyon, Université Lyon 1, Lyon, France Equipe d'Accueil Mixte 4173; Université de Lyon, Université Lyon 1, Hôpital Nord-Ouest Villefranche-sur-Saône, Lyon, France.
authors: Marie-Paule
authors: MP
authors: Gustin
mesh_terms: D018791:CD4 Lymphocyte Count; D003627:Data Interpretation, Statistical; D015658:HIV Infections / Q000188:drug therapy / Q000276:immunology; D006801:Humans; D008137:Longitudinal Studies
keywords: Checking assumptions; grouped-based trajectory models; homoscedasticity; longitudinal data; model adequacy