Think Tank reports provide research on "issues," ranging from climate change to human rights to development financing. They carry the credibility of groups of well-credentialed experts within a field, but at the same time, provide research and conclusions that are in the context of clearly identified perspectives related to "advocacy" for particular causes.
Depending on its "mission", a think tank may purposely, knowingly, and forthrightly have biases or "agendas." Biases are not necessarily bad. Bias can actually be good, as long as the bias is recognized. It's important to identify the bias of the particular think tank as it reflects its mission.
Check these sites to identify biases of specific organizations: