head-on vs rigorous

head-on

adj
  • Direct, abrupt, blunt or unequivocal; not prevaricating. 

  • Of a collision, from the front or in the direction of motion. 

noun
  • A collision from the front. 

adv
  • With the front of a vehicle. 

  • With direct confrontation. 

rigorous

adj
  • Showing, causing, or favoring rigour/rigor; scrupulously accurate or strict; thorough. 

  • Severe; intense. 

How often have the words head-on and rigorous occurred in a corpus of books? (source: Google Ngram Viewer )