Sample size and power calculations
The goal is to enroll 225 individuals into this trial, anticipating 20% drop-out. The trial was powered to detect intervention effectiveness (Aim 1). Assuming 180 participants complete the entire study with a correlation between repeat assessments of at least 0.4, alpha = 0.05 and a two-sided test, we will have over 80% power to detect a difference in slopes between baseline and immediately post-intervention as small as 0.5 standard deviations (SD). If alpha is reduced to 0.025 to account for the two primary outcomes, the minimum detectable difference in slopes is 0.54 SD. Our preliminary studies had a difference in slopes of 0.5–0.6 SD, although we expect the outcomes proposed for this study to be more sensitive to change in behaviors than those used in our preliminary studies.