feedback vs thank-you note

feedback

noun
  • Critical assessment of a process or activity or of their results. 

  • The part of an output signal that is looped back into the input to control or modify a system. 

  • The high-pitched howling noise heard when there is a loop between a microphone and a speaker. 

verb
  • To generate the high-frequency sound by allowing a speaker to cause vibration of the sound generator of a musical instrument connected by an amplifier to the speaker. 

  • To provide informational feedback to. 

  • To convey by means of specialized communications channel. 

thank-you note

How often have the words feedback and thank-you note occurred in a corpus of books? (source: Google Ngram Viewer )