Copyright © The British Psychological Society Reproduction in any form (including the internet) is prohibited without prior permission from the Society The true-change model with individual method effects: Reliability issues Ste ´phane Vautier 1 *, Rolf Steyer 2 and Anne Boomsma 3 1 Centre d’e ´tude et de recherche en psychopathologie, Universite ´ de Toulouse, France 2 Institut fu ¨r Psychologie, Friedrich Schiller University, Jena, Germany 3 Department of Sociology, Faculty of Behavioural and Social Sciences, University of Groningen, The Netherlands The true intra-individual change model is generalized by defining individual method effects. This allows the analysis of non-congeneric test–retest variables assumed to measure a common, possibly (temporally) transient, attribute. Temporal change in the attribute between different times of measurement is modelled by the true-change variable. Individual causal method effects, due to heterogeneity of the measurement methods, account for the imperfect correlation of the true-score variables at each time of measurement. The reliability of the composite scores, at each time of measurement, and the reliability of the difference composite score may be estimated with appropriate coefficients derived from the model. Measurements of daily life tension in adult females serve to illustrate how the model can be used empirically. 1. Introduction Estimation of the reliability of a sum of manifest variables is a long-running methodological research issue in the social and behavioural sciences (e.g. Feldt & Brennan, 1989; Li, Rosenthal, & Rubin, 1996; McDonald, 1999; Raykov, 2001b, 2001c; Stanley, 1971; Steyer & Schmitt, 1990a; Thorndike, 1951; Weiss & Davison, 1981; Zimmerman & Williams, 1998; Zumbo & Rupp, 2004). The congeneric model (Jo ¨reskog, 1971), which subsumes other classical and more restrictive models, inspired a series of papers focused on the estimation of the reliability of a sum of congeneric variables (e.g. Kano & Azuma, 2003; Raykov, 1997a, 1998, 2001b; Reuterberg & Gustafsson, 1992). (Measurement variables are congeneric if (a) the corresponding true-score variables are linearly and positively related and (b) the measurement error variables are uncorrelated * Correspondence should be addressed to Ste ´phane Vautier, Centre d’e ´tudes et de recherche en psychopathologie, Maison de la Recherche, 5 alle ´es A. Machado, 31058 Toulouse Cedex 9, France (e-mail: vautier@univ-tlse2.fr). The British Psychological Society 379 British Journal of Mathematical and Statistical Psychology (2008), 61, 379–399 q 2008 The British Psychological Society www.bpsjournals.co.uk DOI:10.1348/000711007X206826