Recruitment and assessment of the sample. Recruiting and interviewing began in December 1974. Over 220 hours were spent interviewing the sample, which comprised 68 men. Senior officers were preferred because more informed criticism was expected from them. The watchkeeping record of well over two-thirds of the sample amounted to more than ten years' experience; these were masters, chief officers, second officers, uncertificated third officers, senior pilots and, in addition, two lecturers. Many nationalities were represented and the majority held masters' certificates or were attending the college studying for them. The recruitment of serving officers had a rather poor response, numbers being made up by officers on leave. These men came in for a day, a morning or an afternoon, and thus were not exposed to all the systems since such high exposure in a short time might have caused saturation. There was therefore a proportion of the sample which saw only part of the systems though the majority did see it all. There were men who saw just one system, possibly volunteers who did not particularly want to continue, or others who were snatched away from the college by commercial pressures. The approach to these men was important. The sample was exposed to a two-hour lecture period, usually on modern radio aids, and each group was shown the teaching exercises and an explanation of the research objectives as well as the systems.