Professor Swanson developed an index of marital happiness. Several items in Swanson's questionnaire ...

Professor Swanson developed an index of marital happiness. Several items in Swanson's questionnaire were not part of the index (i.e., How many serious quarrels or arguments have you had with your spouse in the last year? If you had your life to live over, do you think you would marry the same person?). Swanson compared the index score of marital happiness with these questions. Swanson was doing
 
  a. index validation.
  b. item analysis.
  c. external validation.
  d. index validation and item analysis.
  e. index validation and external validation.