Why should practice evaluation include consideration of common factors?

Why should practice evaluation include consideration of common factors?
 
  a. Issues in any one of the common factors areas could prevent change work
  from happening or moving forward as dynamically as it could
  b. You cannot evaluate your practice if the common factors are not present so
  you are forced to consider them prior to even beginning a practice evaluation
  c. Considering common factors practice evaluation is consistent with the
  theories outlined in the book
  d. Because your supervisor will expect that you have information to report on
  regarding the consideration of common factors when you are done evaluating
  your practice