Colorado Health Survey: http://www.cdphe.state.co.us/hs/yrbs/childhealth.html
(Unless they were  lying, but they seemed very legit and courteous)

Called numerous times, never left messages when machine picked up.  Other times when I answered, wanted to speak to "the oldest female resident" - asked if I was that person, and wouldn't give any other information when I said no.  Just wanted to know when that person would be available.  I live with my elderly mother and don't feel comfortable letting them speak with her as she is not well and easily confused and may give information that could lead to identify theft.  The caller says they are from the Colordado Department of Health?

They leave no messages and call on the weekends

The began calling in December.  I answered and they said I had been randomly selected to participate in a survey.  The first question they asked (which I don't recall now) seemed so invasive that my gut went on high alert.  I hung up.  They continued calling all day, everyday, morning, noon, night.  I didn't answer it again.  They seem to have finally given up.

The link Curtis provided reflects a survey for parents of children and a health survey.  I have no kids.  This seemed so invasive to me.  They didn't even ask if I had kids.

They started calling 2-13-11, they called every other day and never leave messages. Spoke to them 2-23 and told them not to call again.