A Psychologist Tests the Effect of an Incentive Program (I

Multiple Choice

A psychologist tests the effect of an incentive program (i.e.,positive reinforcement for desired behavior) in a residential treatment facility for delinquent youth.He randomly assigns one building of the large facility to receive the treatment.Residents in a second building serve as a control group.During the course of the one-month study,an event happens in the treatment group that forces full lockdown of the building for one week.The threat to internal validity the psychologist must consider is

A) selection.
B) history.
C) additive effect of selection and history.
D) contamination.