Well, it’s longer than it used to be, per Conor Dougherty in the New York Times:

The median length of time people have owned their homes rose to 8.7 years in 2016, more than double what it had been 10 years earlier.

The accompanying chart shows that “median length of homeownership” used to hover at just under 4 years. That startled me! Doesn’t 4 years seem like a pretty short length of time to own a house?

When I thought about this a little more, I realized I had no idea what this meant. What is the “median length of homeownership” in 2017? Does it mean you go around asking each owner-occupant how long they’ve lived in their house, and take the median of those numbers? Probably not: when people were asked that in 2008, the median answer was 10 years, and whatever the Times was measuring was about 3.7 years in 2008.

Does it mean you look at all house sales in 2017, subtract the time since last sale, and take the median of *those* numbers?

Suppose half of all houses changed hands every year, and the other half changed hands every thirty years. Are the lengths of ownership we’re medianning half “one year” and half “30 years”, or “30/31 1 year” and 1/31 “30 years”?

There are about 75 million owner-occupied housing units in the US and 4-6 million homes sold per year, so the *mean* number of sales per unit per year is certainly way less than 1/4; of course, there’s no reason this mean should be close to the median of, well, whatever we’re taking the median of.

Basically I have no idea what’s being measured. The Times doesn’t link to the Moody’s Analytics study it’s citing, and Dougherty says that study’s not public. I did some Googling for “median length of homeownership” and as far as I can tell this isn’t a standard term of art with a consensus definition.

As papers run more data-heavy pieces I’d love to see a norm develop that there should be some way for the interested reader to figure out exactly what the numbers in the piece refer to. Doesn’t even have to be in the main text. Could be a linked sidebar. I know not everybody cares about this stuff. But I do!

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One reason it would be nice to have links to the data in pieces like these is that I’ve learned not to trust reporters to make sure they’re making apples-to-apples comparisons when looking at changes over time.

It looks like this type of information is collected by the American Community Survey, though I’m not sure if it’s available in this level of granularity. But certainly the “SELECTED HOUSING CHARACTERISTICS” table I pulled up there has things like “Year moved into unit” and what portion of units are owned versus rented. For example, in my state of Illinois in 2015 almost 30% of folks had lived at their current address for less than 5 years.

This kind of issue (the fact that ongoing relationships are expected to last, so the median duration of completed spells is an underestimate) shows up in a variety of places. One of my favorites is this: http://web.stanford.edu/~rehall/Importance-AER-Sep-1982.pdf