Determine meaning To whom does the " other half" in the title refer? Why do you think Riis uses this phrase?

In the book, How the Other Half Lives Riis is referring to immigrants who came to America and are living in New York City. In this book, Riis exposes the poor living conditions of immigrant workers who live in New York City tenements. He calls them "the other half" because this group represents the poor living conditions of millions of immigrant families who suffer on a constant basis in American society. His ultimate goal is to show how this group lives in hopes of invoking change.