Several studies have shown that hospitals are not all equally successful: patients are much more likely to die in some of them than in others. Since the hospitals in the studies had approximately equal per-patient funding, differences in the quality of care provided by hospital staff are probably responsible for the differences in mortality rates.

Which one of the following, if true, casts the most doubt on the conclusion drawn above?


(A) The staff in some of the hospitals studied had earned more advanced degrees, on average, than the staff in the other hospitals.

(B) Patient populations vary substantially in average severity of illness from hospital to hospital.

(C) The average number of years that staff members stay on at a given job varies considerably from one hospital to another.

(D) Approximately the same surgical procedures were performed in each of the hospitals covered in the studies.

(E) Mortality rates for hospital patients do not vary considerably from one region of the country to another.