The role of users in design is diversifying and increasing. Besides product evaluations and idea-generation sessions, users can collaborate in research that aims to find requirements for design by acting as researchers themselves. Earlier studies have addressed a variety of reasons why giving users the role of co-researcher makes sense: users have easier access to the target group (of which they are an active part), speak the same language or are a cost-efficient workforce. A previous study [2] explored the merits and constraints of using children as co-researchers in contextual user research, interviewing peers and family members. This current paper explores two follow-up questions: Can this method also be valuable for the evaluation of design concepts? What differs when other target groups than children become co-researchers? These questions are answered by describing a case in which children, elderly and students performed as co-researchers by interviewing their peers to evaluate design concepts. Comparing this case to the previous one reveals the differences in using co-researchers from different target groups and for different research purposes. We found out that the interfering factor of the co-researcher can have an enriching effect on the research findings.