Over 40,000 people live on the streets of Los Angeles, and counting them takes three nights.

Cities across the U.S. conducted their annual counts of their homeless populations during the last ten days in January. Most only spent one night on the task. While the Department of Housing and Urban Development requires that cities count their total homeless population every other year, LA — second only to New York in the size of its homeless population — chooses to do the tally annually. And because Los Angeles encompasses over 4,000 square miles, the count requires extra time.

This year, over 8,600 volunteers gathered at 166 sites in LA to receive 30 minutes of training before heading out. The count helps the city determine how to distribute about $4.5 billion in funding for homeless services.

Last year’s count found a 23 percent increase in homelessness. But methodology for counting has recently come under criticism. A November study from the non-profit research group Economic Roundtable found that while 2017’s count was close, there’s a “strong possibility” that LA has underestimated its homeless population for years.

“The most basic consequence is that it’s important to know the direction of change,” said Daniel Flaming, president of Economic Roundtable. “Are we making headway or not? So that’s the really big question is what kind of impact are we having.”