When operations are performed purely on prognostic grounds, the decision to operate is based on weighing the risk of the intervention against that of no intervention (conservative treatment). Because the risk of the former is usually immediate and that of the latter is spread out, the time over which the risk is measured becomes important, and this is heavily dependent on the patient’s age. There is no accepted method of comparing risk between conservative and invasive therapies. Furthermore, there is no agreed standard that relates the magnitude of risk to the passage of time. We therefore propose a new approach to help in decision making for both physicians and patients contemplating invasive interventions for purely prognostic indications. This approach is not proposed for operations carried out to relieve symptoms.