Missing from a Census

If your ancestor is missing from a census, don't forget to look in the city directory. My grandmother was listed in the census in 1920 with my 2 1/2 year old mother. She was only listed once even though I thought she died sometime in the early thirties. I found her in the city directory for 1931 and 1932. She was in the same house both times. She lived with another person whom I need to look into. But now I can place her in the timeline that I have for her and can create a research plan.