A researcher is investigating the effects of life review group therapy on depression among elderly residents of assisted care facilities. The researcher identifies two facilities and recruits volunteer participants. In one of the facilities, life review therapy is conducted for 8 weeks. In the other facility, a no-treatment support group takes place for 8 weeks. Depression is measured after the program is administered.
What type of research design is this? What is the greatest threat to internal validity in this type of design?