The consumer price index for December measured inflation at 3.4% from a year earlier — including a 0.3% jump from November. And one particular culprit is keeping inflation elevated: the cost of shelter.

Rent was up 6.5% annually in December, about on par with the price of shelter overall. Those rising housing costs make up more than half of the total monthly increase in consumer prices. But the way the Bureau of Labor Statistics measures shelter makes it more of a lagging indicator, which means rent increases from a while back could have figured into Thursday’s data.

When the Labor Department measures rent prices, it looks at both places that people are just moving into and those where renters are re-upping their leases. 

And if someone is renewing, they’re less likely to pay the market rate, said Steve Reed, an economist with the BLS.

“Because my landlord might not be able by law to make a big adjustment, he might not want to incur the wrath of all his tenants by making a big adjustment,” he said.

Also, the BLS checks in on different rentals in its sample every six months, so if an apartment’s rent goes up it can take a minute for that to be reflected in the data. 

UBS economist Alan Detmeister said all this means the shelter price index in the CPI tends to lag behind other indexes from rental and real estate companies, which show that “market rents, new lease rents, have been running pretty similar to their pre pandemic pace for roughly a year, maybe a little over a year now.”

Whereas the BLS index, he pointed out, shows rents that are increasing closer to twice as fast as they did before the pandemic.

Also, shelter data weighs heavier in the CPI than it does in the personal consumption expenditures price index, the inflation gauge preferred by the Federal Reserve, Detmeister said.

“The fact that any one number can move around your overall inflation basket is a bit problematic. But also it is the biggest cost for most people,” he said.

These are all factors BlackRock investment strategist Gargi Chaudhuri takes into account when she looks at the CPI — or any data point. 

“The broader way to think about it is, let’s look at a bunch of different data. Let’s look at the trend of that data. And let’s not get too caught up in the volatility of one month’s data point,” she said.

Chaudhuri said it’s good to have a healthy respect for what the data is showing, acknowledge its imperfections and not home in too much on any one indicator.