You are creating a data abstraction form to assess the case mix, treatments, and treatment outcome for patients with cancer of the larynx at your center. You plan to perform univariate analysis including Kaplan-Meier curves with log-rank tests and multiple variable analysis to account for effect of disease and age on outcome. At your center some patients have radiotherapy, some have chemotherapy in association with radiotherapy, and some have primary surgery. A specific patient's treatment is based on the extent of disease and overall health. Patients with cancer of the larynx frequently have a variety of comorbid illnesses, often related to smoking and alcohol. For example, Mr. J.B. presented to the coronary care unit with his third myocardial infarction and was found to have a new cancer of the larynx. Due to his chronic obstructive pulmonary disease (COPD), diabetes, and past pulmonary embolus, he is not a candidate for either coronary revascularization or laryngectomy. How do you measure the impact of the comorbid illnesses on the decisions and the outcomes?