It is demonstrated that the popular three-parameter logistic model can lead to markedly inaccurate individual ability level estimates for mixture populations. A theoretically and empirically important setting is initially considered where (a) in one of two subpopulations (latent classes) the two-parameter logistic model holds for each item in a…

The main challenge of ability tests relates to the difficulty of items, whereas speed tests demand that test takers complete very easy items quickly. This article proposes a conceptual framework to represent how performance depends on both between-person differences in speed and ability and the speed-ability compromise within persons. Related…

This article describes an exploration of the distinction between typological and factorial latent variables in the domain of personality theory. Traditionally, many personality variables have been considered to be factorial in nature, even though there are examples of typological constructs dating back to Hippocrates. Recently, some…

Nesselroade, Gerstorf, Hardy, and Ram developed a new and interesting way to enforce invariance at the second-order level in P-technique models, while allowing first-order structure to stray from invariance. We discuss our concerns with this approach under the headings of falsifiability, the nature of manifest variables included in models, and…

Nesselroade, Gerstorf, Hardy, and Ram (this issue) propose to "filter out" idiosyncrasies of dynamic processes at the level of the individual through the application of dynamic factor analysis. The problem that they deal with is that individuals may differ in the items that are "salient" for a given construct, so that the same measurement model…

Nomothetic and idiographic approaches to research have long been in tension. John Nesselroade et al. have been at the forefront of a constructive rapprochement between these traditions. Heretofore their efforts have assumed a common measurement structure across persons. They have primarily focused on modeling relationships within persons, which…

This article presents a rejoinder to Rogosa's (2004) commentary on the author's (Molenaar, 2004) focus article titled, "A Manifesto on Psychology as Idiographic Science." The expert commentary of Rogosa brings up some central issues that require careful evaluation. The basic message of the author's focus article was straightforward: In general,…