From the United States Census Bureau website:
First Use of Questions Asked Only on a Sample Basis
The 1940 Census was the first in which enumerators asked a random sample of the population (roughly 1 in 20 people) an extra set of more detailed questions, including place of birth of their mother and father, mother tongue, veteran status (or whether wife, widow or child of vet), whether deductions for Social Security were made from wages, occupation, industry and class of worker, and, for women who had ever been married, whether they had been married more than once, age at first marriage, and number of children ever born. All in all, 34 questions were asked of all households, with another 16 asked of the one in 20 sample.
I sure hope some of my ancestors were among that random sample!