Data analysis
depression
SESSION
Data were analyzed as intent-to-treat; therefore, all service members eligible for study participation (Both longitudinal (preprogram, postprogram, 3-month follow-up) and within session (presession, postsession) analyses were conducted with MLM using a step-up model building process. Logical covariance matrices were compared and selected based on model fit according to Akaike information criterion with respect to the number of parameters specified. All final multilevel models used restricted maximum likelihood to account for missing data.For longitudinal analyses, the intercept was set as a random effect of subject with a diagonal covariance matrix. Time was a repeated effect with a covariance matrix of scaled identity. Piecewise analysis was used with longitudinal MLM models to best account for different independent variables in the intervention and follow-up periods (e.g., number of sessions attended in each time frame). Fixed effects were specified as follows: time (0 = preprogram, 1 = postprogram in pre- to post-models; 0 = postprogram, 1 = 3-month follow-up in follow-up models); intervention condition (0 = Hike Therapy, 1 = Surf Therapy); concurrent pharmacotherapy for depression (0 = no, 1 = yes); concurrent psychotherapy for depression (0 = no, 1 = yes); physical activity level (0 = high, 1 = moderate, 2 = low); and number of exercise therapy sessions attended (continuous total). Each fixed effect was also used in an interaction term with time.For within-session analysis, intercept, time (pre- to postsession), week of session, and a crossed effect of Time × Week of Session were set as random slopes by subject with a first-order autoregressive covariance matrix. Time × Week of Session was set as a repeated effect of subject and used a compound symmetry covariance matrix. Fixed effects included time (0 = presession, 1 = postsession); intervention condition (0 = Hike Therapy, 1 = Surf Therapy); concurrent pharmacotherapy for depression (0 = no, 1 = yes); concurrent psychotherapy for depression (0 = no, 1 = yes); physical activity level (0 = high, 1 = moderate, 2 = low); and week of exercise session (continuous week number). All fixed effects were also used in individual interactions with time.