irregularity vs variability

irregularity

noun
  • The state or condition of being irregular, or the extent to which something is irregular. 

  • A violation of rules. 

  • An object or event that is not regular or ordinary. 

  • An instance of being irregular. 

  • Irregular bowel movement (e.g. diarrhea or constipation). 

variability

noun
  • The state or characteristic of being variable. 

  • The degree to which a thing is variable. In data or statistics this is often a measurement of distance from the mean or a description of data range. 

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