Anchor: If case studies can vary so greatly, how can we assess their usefulness for evaluation?
Entailed: If case studies vary greatly how do we assess their usefulness?
Contradiction: Case studies don't vary 
Contradiction: The policeman is at the police station in the canteen having lunch
Contradiction: The man is outdoors.
Contradiction: The student is alone in his bedroom.
Contradiction: Their group is involved in joint ventures with Ford and Nynex.