Problem
Loriann wants to see if mindfulness will reduce obsessive-compulsive behaviors. Using a sample of 50 people with obsessive-compulsive disorders (OCD), she randomly assigns them to receive a mindfulness intervention (n = 25) or traditional cognitive behavioral therapy (CBT; n = 25). After 10 weeks of treatment, she measures the severity of their OCD symptoms. However, in a discussion with the therapist giving the mindfulness interventions, she learns that several people in the CBT group attended the mindfulness training too. What threat to validity should she be concerned with?