ballad vs folk story

ballad

noun
  • A kind of narrative poem, adapted for recitation or singing; especially, a sentimental or romantic poem in short stanzas. 

  • A slow romantic song. 

verb
  • To compose or sing ballads. 

folk story

noun
  • Synonym of folk tale. 

How often have the words ballad and folk story occurred in a corpus of books? (source: Google Ngram Viewer )