The validity scales of the group MMPI have 2 weaknesses: the F scale does not detect random answers on the back page of the IBM answer sheet, and it does not separate examinees who get high scores from random responses from those who may hold certain delusions. "Both these problems are partially solved by the Tr scale which discovers inconsistencies of response to the duplicated items in the booklet form." (PsycINFO Database Record (c) 2006 APA, all rights reserved)