How much incentives is too much?

While planing the research for a conference paper, a question remains present to us. The study involves three simple steps: (1) initial participatory interview; (2) participants interaction with a home device during a couple of weeks; and (3) a final interview to collect feedback.

For this type of studies, companies are paying an average amount of money to recruit and motivate participants. However, there is an information lack regarding what should be the number of incentives, as well as the corresponding amount, for study purposes. So, our questions are as follows.

List of questions to be answered:

• How many incentives is too much?

• Is there any State-Of-The-Art to follow the above question?