lower class vs working class

lower class

noun
  • A class of people in a society characterized by low incomes, low level of education, high unemployment and, as a result of these, a low social status. 

working class

noun
  • The social class of those who perform physical work for a living, as opposed to the professional or middle class, the upper class, or others. 

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