7. What are the independent and dependent variables? In response to an increase in the number of pilot deviation (PD) runway incursions (RIs) among general aviation (GA) pilots 65 years or older, an aviation human factors researcher hypothesizes that a plausible explanation is memory related: She be- lieves that older GA pilots' short-term memory is failing, and they are not remem- bering ATC instructions. After seeing a television ad for a supplement that claims to improve memory, she decides to test the effect of the supplement on older pilots. Her rationale is that if pilots' memory capacity could be improved, then this might result in a decline in PD RIs. She randomly selects a sample of GA pilots who are 65 years or older and has them take one capsule of the supplement daily for 6 months. Using a simple memory test that requires memorizing a sequence of 12 digits in the order in which they are presented, she found that the time it takes to memorize these digits is normally distributed with a mean of 72 seconds and standard devia- tion of 18 seconds (the longer the time, the less the memory capacity).