On Friday, the U.S. National Archives will unlock the complete records of the 1950 Census. These files describe more than 150 million people, one by one. They bear the handwriting of a small army of Americans (around 150,000 in all) who scoured the nation, city and country, and even the recently sprouted suburbs, asking more than 20 questions of every person.

Today, we can see the 3 billion answers to those questions as “big data,” possibly yielding new discoveries about the past. Scholars will probe the data set to study social mobility, residential segregation or the everyday lives of Americans at mid-century. Countless genealogical investigators will mine the records for information about their kin or communities.

These records are of particular significance because they are the last of their era. In subsequent decades, the government embraced more fully a statistical technique that allowed it to ask far fewer questions of most individuals. Censuses from 1960 on would still record nearly every person, but with far less depth or detail. The 1950 census therefore stands as a testament to the value of an expansive, exhaustive census and all that it saves for posterity.

The Constitution instituted the U.S. census as part of the machinery of democratic governance, tying representation to population. It called for an “actual Enumeration” to determine what proportional share of the House of Representatives each state would be due. In meeting this goal, the first census of 1790 did little more than count heads.

That first enumeration only tallied the numbers of White men (split into two age groups), White women, those the census referred to as “all other free persons” and enslaved people. It excluded Native Americans and credited enslaved people as only three-fifths of a person for apportionment. Only heads of household were named in the ledgers, and so only their details were recorded.

Through the 19th century and into the 20th, the scope of the census expanded. Congress added new questions, hoping the answers would inform its deliberations. In 1850, as the census became more curious, it also began recording separate answers for each individual person in a household, though only free people were named.