Marcus conducted an investigation to evaluate the effectiveness of a new social skills training programme for violent offenders. He allocated the first 20 offenders who volunteered for the scheme to the treatment group, and another 20 offenders who were transferred to the facility later to the control group.
Explain one weakness of Marcus's investigation into the new social skills training programme.
51 exam-style questions on Edexcel A Level Psychology Research methods and skills, covering Research methods and skills. Each one has a worked solution and a mark scheme showing where the marks go.