Factoring at the Individual Level: Some Matters for the Second Century of Factor Analysis
John R. Nesselroade
Ohio State University

Despite a somewhat controversial beginning in psychology, fitting the common factor model to multivariate time series data has played an influential role in advances such as the trait-state distinction and in apprehending the nature of relatively short-term, intra-individual variability. As seen in Michael Browne's presentation, innovations of the past couple of decades have helped to clarify the rationale of the general approach and have greatly strengthened its appeal for students of intra-individual change and variability. As the rationale for individual level factor analysis develops, old concerns such as the idiographic-nomothetic debate are surfacing in this new context. These concerns, in turn, suggest that concepts such as factorial invariance that have been uncritically incorporated into individual level factor modeling need to be evaluated and perhaps re-interpreted. Some key issues are identified and discussed with a view to ensuring that factor analytic modeling will continue to be the productive activity it has been for the past 100 years.