Controlled experiments to test new military personnel policies on a small scale before they are implemented have many advantages over other techniques for evaluating the potential effects of these policies. However, case studies of recent experiments in the Army and Air Reserve Forces demonstrate the hazards of conducting such studies unless precautions are taken to assure the validity of the experimental results. The Army's experiment to test the feasibility of shortening the term of enlistment in the reserves provides a particularly good example of how not to conduct a field study of this type. The lessons learned from this and other experiments underscore the necessity of following certain guidelines in planning and conducting such tests in the future. 44 pp.