Practical implications
Our findings can help to improve future PA behavioral interventions. Trials should be aware of non‐adherence, especially in the beginning of a behavioral intervention. Our results showed that the attitude towards behavioral change was established in the first month of the intervention. Furthermore, contamination needs to be considered in studies that recruit participants willing or motivated to enhance their PA. Power calculations are a critical first step when a trial is planned; expected effect sizes could be modeled on the assumption of a lower increase in PA than expected among the intervention group because of non‐adherence, a parallel increase in PA among controls, and dropouts thereby reducing the actual delta between the two arms. Taking our findings of non‐adherence and contamination together, alternative study designs like attention control groups, SCEDs or step‐wedged designs could be valuable alternative options. One also needs to be aware of the problem of self‐selection bias in PA behavior interventions, particularly in studies of survivors where strategies should be applied to screen and potentially exclude already highly active participants.