In the pilot studies, we could not verify the efficacy of each incentive or procedure separately because different types of incentives were often offered together. We did, however, ask participants to rank the factors that influenced their participation. Among the most influential factors reported by subjects recruited through December 2003, "desire to help medical research" (78%), "reassured by the family physician" (53%), and "possibility to participate from home" (44%) were the most frequent "very high" scores. "Receiving compensation", "obtaining information by calling a toll-free number", and "receiving the letter from the mayor of Milan" were the factors with the most frequent, "very low" scores (61%, 47%, and 40%, respectively). The majority of subjects were not aware of the advertisements about the study that appeared on the local TV or newspapers. These data are relevant to future studies, with the caveat that they are based on self-reporting in one cultural setting, and need to be evaluated by direct comparison.
