FiveThirtyEight is a guilty pleasure for any data junkie, and now it’s turned its gaze on homeownership.

The question of how many homeowners have a home without a mortgage is more complicated than you might think. Take a look.

...home ownership can have big consequences for the economy, because 56% of all the housing units in America (that includes trailers, apartments and houses) are owned by the people who live in them.

According to last year’s American Community Survey, one in three of those owner-occupied housing units doesn’t have any mortgage left to pay.

You and I are talking about slightly different things here, though, John: You’re asking about homeowners and I’m providing data on owner-occupied housing units. That’s because the American Community Survey summarizes data about the country’s 132.8 million housing units, not the individuals who own them.

If these different types of housing tend to have different numbers of people living in them (that seems pretty plausible to me) then these percentages about housing units don’t exactly translate to percentages about people. In other words, this data doesn’t show that 20% of Americans are homeowners who’ve paid off their mortgage debt, only that 20% of housing units are owner occupied with no mortgage left to pay.