“The Tea Bag Experiment: More Evidence on Incentives in Mail Surveys,” Mike Brennan, Janet Hoek, and Philip Gendall, International Journal of Market Research, vol. 40, no. 4, 1998, pp. 347–52. The authors report [AIR 16:2]:
This paper reports the results of a study which compared the effectiveness of a tea bag and a $1 coin as prepaid incentives in a mail survey of the general public. The tea bag had no effect on response rate.