Random Assignment with a Very Large Sample of Clients to Two

Multiple Choice

Random assignment with a very large sample of clients to two different groups of practitioners, one of which provides an innovative intervention and the other of which provides treatment as usual, is sufficient for ruling out which of the following explanations for results showing that the experimental group's outcome was significantly better than the control group's outcome?

A) Lack of practitioner equivalence.
B) Differential attrition rates.
C) Obtrusive observation.
D) All of the above.
E) None of the above.