Evaluate the relative importance of causes of internal migration within the United States in the period from 1900-1970.

Evaluate the relative importance of causes of internal migration within the United States in the period from 1900-1970.
Evaluate the relative importance of causes of internal migration within the United States in the period from 1900-1970.

That sounds like a research question! There’s a research guide for US History that could be a useful starting point: https://libguides.bc.edu/ushistory. And the Reference Desk just across the lobby is staffed by librarians who can also help you answer this question.