Are you using the 1940 or 1950 samples?

Find out about sample-line characteristics
 

Are you using the 1860, 1870, 1940, 1950, or 1990 samples?

Find out about sample weights
 

Are you investigating household structure, household size, or family interrelationships?

Find out about variations in group quarters definitions
 

Are you using geographic codes below the state level in the period from 1940 to 1990?

Find out about geographic coding and comparability