COMMENT ON HOW MANY CAUSES ARE THERE OF AGING-RELATED DECREMENTS IN COGNITIVE-FUNCTIONING

Salthouse (1994) introduces a new type of correlation, termed the quas i-partial correlation (QPC). The QPC reflects the amount of age-relate d variance that is shared by two variables. In this commentary it is a rgued 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 i nstantaneous. Where these assumptions are reasonable, QPC's are a high ly valuable research tool. It is also argued that the ''causes'' of de cline identified by Salthouse might be termed ''dimensions'' of declin e instead, because the analysis did not include independently assessed causal variables. Finally, it is suggested that age is unsatisfying a s a predictor of change across the lifespan. (C) 1994 Academic Press, Inc.