Within-subjects, parametric manipulations to investigate aging.

Parametric manipulation of experimental variables is important for both practical and theoretical reasons. Practically, age-related impairments in memory may be detected only at certain levels of difficulty. If the task is too simple, age-related cognitive impairments may not be sufficient to alter performance. If the task is too difficult, age-related cognitive impairments may not add significantly to the poor performance of younger controls. Thus, some intermediate level of difficulty is necessary to produce behavioral evidence on underlying age-related cognitive changes. Theoretically, parametric manipulations are important to identify the psychological processes responsible for the behavioral alterations. For example, consider the common finding of an age-related impairment in a delayed conditional discrimination task. If that impairment is produced by a primary deficit in recent memory, then the magnitude of the age-related impairment should increase with the length as the delay interval is increased, producing a statistical interaction between the age delay interval. Both the practical and theoretical considerations are illustrated with specific experiments, and have implications for the types of experimental design that can be most effective in demonstrating and explaining age-related changes in learning and memory.