Kerr J, Rosenberg DE, Nathan A, Millstein RA, Carlson JA, Crist K, Wasilenko K, Bolling K, Castro CM, Buchner DM, Marshall SJ. Applying the ecological model of behavior change to a physical activity trial in retirement communities: description of the study protocol. Contemp Clin Trials. 2012 Nov; 33(6):1180-8.