Explain the potential problem of the following study and propose an alternative way to prevent the problem.
As a drug manufacturer, you're interested in studying the effectiveness of a new drug for headache relief. You place an ad in the newspaper recruiting "headache sufferers" who want to volunteer for the study. At the beginning of the study, you question each participant about the frequency and duration of headaches. Then you send the sufferers home with a week's supply of the new medicine. A week later, you ask each participant the same questions about their headaches.