a physician hypothesized that a low-dose aspirin regimen beginning in a person's 40s could reduce the likelihood of developing alzheimer's disease. with proper consent and protocols in place, she established two groups of 40-year-old patients. each group consisted of 1,000 patients. the patients in one group were asked to take a low-dose aspirin regimen for three decades. every year for the next 30 years, the physician assessed all patients for symptoms of alzheimer's. which is the dependent variable in the physician's experiment?