Proportional slowing and addition speed in old and young adults.

An addition and copying task was used to compare processing speed in young and old adults. Consistent with previous studies, no age differences in the problem-size effect were observed (Geary & Wiley, 1991). However, the old adults were slower overall, and an analysis of the distributions of old and young individuals indicated that the form of this slowing was proportional. These analyses also demonstrated that proportional slowing was uniform in the old adults, such that the fastest and slowest old individuals were slowed by the same factor. Because the regression of old-young mean response times can be insensitive to differential age effects, comparisons of old and young distributions are recommended to support claims regarding proportional slowing and uniformity of age effects across individuals. Finally, the results suggest that requiring Ss to initiate a new operation produced a larger age effect than requiring Ss to repeat an operation.