Do participants' preferences for mode of delivery (text, video, or both) influence the effectiveness of a web-based physical activity intervention?

Background: In randomized controlled trials, participants cannot choose their preferred intervention delivery mode and thus might refuse to participate or not engage fully if assigned to a nonpreferred group. This might underestimate the true effectiveness of behavior-change interventions. Objective: To examine whether receiving interventions either matched or mismatched with participants’ preferred delivery mode would influence effectiveness of a Web-based physical activity intervention.