The authors of an IRB approved study report that, through the use of participation incentives, 89% of the eligible cases agreed to participate and 60% of the 1060 controls approached agreed to participate. When you evaluate this paper you:.

d. Are concerned about non response bias among the controls

Explanation:

Participation incentive is an extrinsic motivator, it is a reward given to encourage participation of people for a certain thing, like a research in this case.

They are particularly useful when the desire to participate in the research for the sake of participating is weak.

From the question, being concerned about no response bias among the group used as control for the research is important because they were not given incentives for them to participate, where as incentives were given to the other group of people (eligible cases)