Arthritis study

Received a letter in the mail asking me to participate in a study for a cash reward. After reviewing other people's responses on this site, it appears this company collects your information over the phone and then tells you there is no study in your area so they don't have to pay you. That sounds like a scam to me. Not to mention the letter says they are looking for study participants in my town (which they name in the letter). I live in a very rural town of about 3,000 people. Yes, this is just the place any large research firm would choose to do a study on arthritis!