The Advantage of Using Outside Independent Observers or Pre-Post Test

The advantage of using outside independent observers or pre-post test comparisons to evaluate a client's progress is

A) they take less time and are less costly.
B) they provide a valid prediction of how the client will behave in real life.
C) they increase the objectivity of ratings over those given by the people involved in treatment.
D) they insure that there is no "regression to the mean" phenomenon.