post mortem vs post-mortem

post mortem

noun
  • An investigation of a corpse to determine the cause of death. 

  • Any investigation after the conclusion of an activity, particularly when said activity produces an unwanted outcome. 

adj
  • Having been inflicted or having occurred after death. 

adv
  • Occurring after death. 

post-mortem

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