low-grade vs subnormal

low-grade

adj

  • Of inferior quality. 

subnormal

adj

  • Less than normal. 

  • denormal 

noun

  • A person whose abilities are less than normal. 

  • That part of the axis of a curved line which is intercepted between the ordinate and the normal. 

How often have the words low-grade and subnormal occurred in a corpus of books? (source: Google Ngram Viewer )