  The predictions from an accurate prognostic model can be of great interest to
patients and clinicians. When predictions are reported to individuals, they may
decide to take action to improve their health or they may simply be comforted
by the knowledge. However, if there is a clearly defined space of actions in
the clinical context, a formal decision rule based on the prediction has the
potential to have a much broader impact. Even if it is not the intended use of
a developed prediction model, informal decision rules can often be found in
practice. The use of a prediction-based decision rule should be formalized and
compared to the standard of care in a randomized trial to assess its clinical
utility, however, evidence is needed to motivate such a trial. We outline how
observational data can be used to propose a decision rule based on a prognostic
prediction model. We then propose a framework for emulating a prediction driven
trial to evaluate the utility of a prediction-based decision rule in
observational data. A split-sample structure can and should be used to develop
the prognostic model, define the decision rule, and evaluate its clinical
utility.
