do something off the top of one's head vs improvise

do something off the top of one's head

improvise

verb

  • To make something up or invent it as one goes on; to proceed guided only by imagination, intuition, and guesswork rather than by a careful plan. 

How often have the words do something off the top of one's head and improvise occurred in a corpus of books? (source: Google Ngram Viewer )