Advocates of evidence-based policymaking (EBP) often argue that policy decisions are improved when they are informed by rigorous and accurate scientific evidence. However, some critical authors argue that public policies cannot be decided on technical evidence alone. They stress that calls to simply ‘base’ policy on evidence risks ignoring the fundamental importance of politics as a mechanism to debate and choose between multiple competing social concerns, and further risks imposing de facto policy priority on those concerns which have been measured, or those which are conducive to measuring in particular ways. At times, debates between these groups appear to paint an intractable difference of opinion on the role that evidence can or should play in policymaking. However, here we argue that both sides have valid concerns to consider, yet their concerns are very different in nature. For champions of evidence, there is a problem with the politicisation of science – the ways that political interests appear to drive the misuse, manipulation, or cherry picking of evidence to promote political goals. This can otherwise be defined as a concern over technical bias in the use of evidence – evidence utilisation that does not follow principles of scientific best practice (which can include invalid uses of individual pieces of evidence, as well as failing to systematically include all the relevant evidence that best answers a particular question) and which therefore leads to poorer policy outcomes than would otherwise be possible. The critical policy perspective, on the other hand, points to the problems caused by the depoliticisation of politics – in particular the ways in which social values can be obscured or marginalised through the promotion of certain forms or bodies of evidence. This is also a form of bias, but can be alternatively termed issue biasto capture how evidence utilisation can shift the political debate to particular questions or concerns in a non-transparent way. The first form of bias broadly reflects the value of scientific fidelity, while the second broadly reflects the value of democratic representation. This brief defines these concepts and explores the political origins of these different forms of bias in order to help move beyond the debates between evidence champions and critical perspectives, as well as to help guide efforts to avoid bias or mitigate its impact.