Comment on "How Many Causes Are There of Aging-Related Decrements in Cognitive Functioning?"

Salthouse (1994) introduces a new type of correlation, termed the quasi-partial correlation (QPC). The QPC reflects the amount of age-related variance that is shared by two variables. In this commentary it is argued that the QPC is a model where age is a suitable proxy for time, change over time is linear, and causation is instantaneous or nearly instantaneous. Where these assumptions are reasonable, QPC′s are a highly valuable research tool. It is also argued that the "causes" of decline identified by Salthouse might be termed "dimensions" of decline instead, because the analysis did not include independently assessed causal variables. Finally, it is suggested that age is unsatisfying as a predictor of change across the lifespan.