Dr. Alvarez is studying the effects of cannabidiol (CBD) on…
Dr. Alvarez is studying the effects of cannabidiol (CBD) on pain sensitivity. She recruits 40 patients with chronic pain and wants to assign half of them to receive CBD and half to receive placebo before completing a pain sensitivity test. However, Dr. Alvarez lets patients choose whether they want CBD or placebo, rather than randomly assigning them. Which threat to internal validity would this introduce?