Yes, as the title of the post suggests I've been doing a lot of reading and writing about unconditional incentives. In 2009, a research team here pilot tested our postal survey mailing method and less than 60% of the women who were contacted returned their questionnaire. That's a problem when you want to follow women for five years to collect data that you'd like to generalize to all the New Zealand women in the target age range.
So, they included little tokens of appreciation - University of Otago pens with the study name, individually wrapped tea bags and conditional lottery draws - in the questionnaires to increase the response rates.
Oh, and a few randomly selected people received cold hard cash. $$$
Guess who was more likely to return her questionnaire?