Background Amid the calls for ‘more public health evidence’, we also need simple understandable methods of determining when more research really is needed. This paper describes a simple decision aid to help policymakers, researchers and other decision makers assess the potential ‘information value’ of a new public health randomised controlled trial.

Methods The authors developed a flow chart to help make explicit (1) the user's information needs, (2) the intended use of the new information that the study will produce, (3) the added value of the evidence to be derived from the new study and (4) the levels of precision, bias and generalisability required by the user.

Results The flow chart is briefly illustrated, first in generic form and then in a worked example, showing how it may be used in deciding whether a new study should be commissioned to evaluate the health impact of allowing motorcycles to use bus lanes in London.

Conclusions In this paper, the authors have presented a flow chart for enacting an informal ‘Value-of-Information’-like approach to deciding when a new public health evaluation is needed. The authors do not suggest that the flow chart approach is technically the equivalent of Value-of-Information methods. Nonetheless, it represents a valuable perspective and process to adopt, and this structured approach will be more revealing than an unstructured thought experiment as the basis for decisions about a new study. To aid in its development as an effective tool, we invite users from a variety of perspectives and contexts to review it, to use it in practice and to send us their comments.