ditty vs folk song

ditty

noun
  • A short, simple verse or song. 

  • A saying or utterance, especially one that is short and frequently repeated. 

verb
  • To sing; to warble a little tune. 

folk song

noun
  • A song in the style of folk music, a modern genre of popular music. 

  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • folk music 

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