Severe post-operative bleeding can lead to adverse outcomes for cardiac surgery patients and is a relatively common complication of cardiac surgery. One of the most effective drugs to prevent such bleeding, aprotinin, has been withdrawn from the market due to concerns over its safety. Alternative prophylactic drugs which can be given to patients to prevent bleeding can result in significant side effects and are expensive. For this reason it is difficult to make a clinical or economic case for administering these drugs to all cardiac surgery patients, and the prevailing view is that their use should be targeted at patients considered to be at relatively high risk of post-operative bleeding. However, there is currently no objective method for identifying such patients. Over the past 7 years, a team of clinicians and researchers at Papworth Hospital has collected data concerning post-operative blood loss for each cardiac surgery patient, totalling 11,592 consecutive records. They approached a team of operational researchers (MU, ACP, BR) with extensive experience of developing clinical risk models with the aim of devising a risk stratification scheme that could potentially be used to identify a cohort of higher risk patients. Such patients could be treated with the available prophylactic drugs or recruited to studies to evaluate new interventions. This paper is intended to describe the Operational Research process adopted in the development of this scheme. A concise description of the scheme and its clinical interpretation is published elsewhere.