In public health, as in other fields, there has been much discussion about the type of evidence we need for decision making. In particular, there has been much debate about how the production of trials can be increased. However, this is not the only challenge and perhaps not even the main one. A better understanding is needed of why trials are not conducted, and what sort of evidence is used in their place. More generally, researchers need a better understanding of the ways in which other forms of evidence than trials are informative to decision makers.