A study examines the effect of a new form of psychotherapy o

A study examines the effect of a new form of psychotherapy on Post-Traumatic Stress Disorder (PTSD.) A group of PTSD patients complete a survey measuring the severity of their symptoms. They are then given the new form of psychotherapy for 3 weeks. After the three weeks, they again complete the survey of symptom severity. Of the following, circle the one that is most likely to represent a threat to the internal validity of this study.

a. Selection

b. Testing

c. History

Solution

b. Testing