Recruitment experience in the Nocturnal Oxygen Therapy Trial.

The Nocturnal Oxygen Therapy Trial was a randomized controlled clinical trial sponsored by the National Heart, Lung, and Blood Institute and carried out by six clinics. The primary objective was to compare, in patients with advanced chronic obstructive pulmonary disease, the effectiveness of 24 hours of oxygen administration to that of 12 hours of oxygen administration including the patient's usual period of sleep. Some patients entering the period of baseline observation were not eligible for randomization at the end of the baseline period. Such attrition is discussed relative to setting goals for the number of patients to enter such an observation period. The impact of an enrollment rate less than what was originally projected is discussed relative to changes in the eligibility criteria and relative to the decision as to whether to extend the recruitment period.