One big component of the Consumer Price Index — housing — stayed stubbornly high in December. The shelter index rose .8% from the month before. It was up 7.5% year-over-year, even though there’s been a big slowdown in the housing market, with both rents and home prices falling in recent months.

Quick refresher: The government measures housing inflation by looking at rent and what’s called owners’ equivalent rent:

“What a homeowner’s home might rent for, if it were to be a rental,” explained Igor Popov, chief economist at Apartment List.

Both measures in the CPI rose by .8% last month, but Popov said that if you look at spot rents — what landlords were charging on new leases — “in our December data, rents fell nationally .8%,” Popov said. “Incidentally, that’s the opposite of what shelter inflation did.”

Spot rents have been falling for the past four months, he added, but that’s going to take a while to show up in the CPI estimates. “Because they’re meant to represent what average Americans are paying for their rent, and average Americans don’t renew their leases every single month,” Popov explained.

Similarly, the lag between the Case-Shiller home price index and owners’ equivalent rent can be 12 to 16 months, said economist Mickey Levy at Berenberg Capital Markets. He expects both rent and owners’ equivalent rent to go up a bit further in the CPI in the near term.

“But then they will subside and come down fairly significantly, you know, beginning in the second half of the year,” Levy said.

Given how much weight shelter carries in the overall CPI — about a third — I wondered if that lag was making inflation look worse than it is?

Jay Parsons, chief economist for the real estate software and analytics firm RealPage, said there’s a live debate about that among economists.

“Bottom line is that, you know, the rental market has dramatically cooled off,” he said. “And the CPI, at this point, is the only data set that’s not reflecting that.”

And it’s funny. When I called Apartment List’s Igor Popov this morning, he reminded me that we had the inverse of this conversation almost exactly one year ago, when the CPI wasn’t capturing the sharp increases in rent. What a difference a year can make.