approval vs niggle

approval

noun
  • Something mailed by a seller to a collector to match their stated interests; the collector can approve of or return the item. 

  • An expression granting permission; an indication of agreement with a proposal; an acknowledgement that a person, thing, or event meets requirements. 

  • An expression of favorable acceptance and encouragement; a compliment that also condones. 

niggle

noun
  • A minor complaint or problem. 

verb
  • To use, spend, or do in a petty or trifling manner. 

  • To dwell too much on minor points or on trifling details. 

  • To fidget, fiddle, be restless. 

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