in other words vs that is to say

in other words

adv

  • used to introduce an explanation, simplification, or clarification 

prep

  • Stated or interpreted another way; Used to introduce an explanation, simplification, or clarification. 

that is to say

adv

  • In other words. Used to introduce a clarification, simplification, or explanation 

How often have the words in other words and that is to say occurred in a corpus of books? (source: Google Ngram Viewer )