What is not a common feature of both the mmpi-2 and cpi? both interpret answers using categories. both have groups of questions that check for validity. both have ways of checking to see if people lied in their answers. both check for serious personality disorders.

The correct option is this: BOTH CHECK FOR SERIOUS PERSONALITY DISORDER.
CPI is designed to assess normal personality while MMPI are personality instruments that are especially designed for some settings such as educational, employment and medical settings. CPI does not check for serious personality disorder only MMPI does that.