Yesterday, I talked about utilizing incentives for longer than typical studies, or longitudinal surveys.
In addition to splitting our total incentive amount throughout the entire effort rather than all at one time, another key issue to think over is whether you want your incentives to be conditional or unconditional.
As we discussed in Part I, survey research that looked at incentives for longitudinal surveys was published in the Journal of Official Statistics (JOS). One idea for using incentives in longitudinal efforts is to make your first incentive offer conditional (that is, participants only receive the incentive AFTER they provide data) and the second and subsequent offers unconditional (providing the incentive regardless of whether or not participants provide information for the effort).
Mixing up the way you offer incentives for your longitudinal efforts may decrease the amount of incentives that are "lost." That is, money or products that are given out to those who do not respond at all. A lot depends on the survey response rate you expect at your first wave (and to a lesser extent, your data collection method).
As you can see, there's a lot to think about when providing incentives to participate in your longitudinal survey, but if planned out, your research can benefit greatly from them.