It wasn't until 1980 that the government decided to collect Census data by mail-in survey. The shift to a survey had dramatic effects on at least one Census category: race.
Before 1980, Census enumerators categorized people into racial groups based on their appearance. They did not ask respondents how they characterized themselves. Instead, they made a judgment call, drawing on explicit instructions given to the Census takers. On a mail-in survey, however, the individual self-identified. They got to tell the government what race they were instead of letting the government decide. There were at least two striking shifts as a result of this change:
First, it resulted in a dramatic increase in the Native American population. Between 1980 and 2000, the U.S. Native American population magically grew 110%. People who had identified as American Indian had apparently been somewhat invisible to the government.
Second, to the chagrin of the Census Bureau, 80% of Puerto Ricans choose white (only 40% of them had been identified as white in the previous Census). The government wanted to categorize Puerto Ricans as predominantly black, but the Puerto Rican population saw things differently.
I like this story. Switching from enumerators to surveys meant literally shifting our definition of what race is from a matter of appearance to a matter of identity. And it wasn't a strategic or philosophical decision. Instead, the very demographics of the population underwent a fundamental unsettling because of the logistical difficulties in collecting information from a large number of people. Nevertheless, this change would have a profound impact on who we think Americans are, what research about race finds, and how we think about race today.