Behavioural Science Section Gerontology 2002;48:13–17 Correct Identification and Explanation of Age-Related Changes Are Always Possible Peter C.M. Molenaar University of Amsterdam, The Netherlands Peter C.M. Molenaar University of Amsterdam Roetersstraat 15, Room 502 NL–1018 WB Amsterdam (The Netherlands) E-Mail op_molenaar@macmail.psy.uva.nl ABC Fax + 41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2002 S. Karger AG, Basel 0304–324X/02/0481–0013$18.50/0 Accessible online at: www.karger.com/journals/ger Key Words Age-related changes, identification W Research design W Longitudinal design Abstract In this commentary, it is argued that the definition of the time or the age variable in the models presented by Ho- fer and Sliwinski is ambiguous, and a standard interpre- tation of this time of age variable is proposed instead. Some simulation studies are presented, the results of which indicate that, even with age-heterogeneous cross- sectional data, correct identification of the true sources of age-dependent variation is possible. An important qualification, based on new methodological work in pro- gress, is given of the scope of longitudinal designs. It is concluded that all kinds of age-related changes are po- tentially interesting and can be correctly identified and explained. Copyright © 2002 S. Karger AG, Basel Introduction I have read with great interest the paper by Hofer and Sliwinski [1] (to which I will henceforth refer to as ‘the authors’). It deals with some of the methodological issues when using cross-sectional designs in order to investigate age-dependent changes. In what follows I will present a critical discussion of various aspects of the paper. Analyt- ic discourse necessarily has a critical undertone. There- fore, I would like to stress at the outset that the authors deserve praise because they make a fundamental state- ment and raise important questions about the informa- tion which can be obtained from age-heterogeneous cross- sectional data. This commentary is organized as follows. First I will shortly discuss the basic equation 1 given in the paper. From that discussion I will draw some conclusions con- cerning the correctness of the remaining equations. Sec- ond, I will present some results from simulation studies based on the basic equation 1 given in the paper. The results of these simulation studies indicate that, even with age-heterogeneous cross-sectional data, correct identifica- tion can be made of the true sources of age-dependent variation. I also will consider the remarks about partiall- ing out of age made by the authors. Third, I will comment on the statements made by the authors about longitudinal analysis. These comments will include new methodologi- cal developments that qualify the type of information which can be obtained even from what is regarded as the most powerful design to study within-subject variation, namely the longitudinal design. In closing this commenta- ry, some general remarks will be made concerning designs with several homogeneous age groups and the relation thereof with the age-homogeneous cross-sectional and sequential designs favored by the authors. I end with a general evaluation of the paper.