The Gulf Coast of Southern Mississippi, a Continuum-of-Care community that spans six counties, is another region that reached functional zero for veteran homelessness in 2015. Today, any veterans who fall into homelessness there are linked almost immediately to service providers who help them get Veterans Affairs benefits and into housing in less than 30 days. "We're working to get back to our 2015 average of 11 days," said Mary Simons, the executive director of Open Doors Homeless Coalition, which leads collaborative efforts in the region.

In the Built for Zero methodology, after communities build a local team and are sharing real-time data, the next milestones are achieving steady reductions in homelessness, getting to zero and sustaining it. As they work more effectively, they also focus on other dimensions like youth and family homelessness, which are very different problems and require different responses.

The Gulf Coast community has been sustaining zero homelessness for veterans for two years. It has had to be continually vigilant. "In the middle of 2016, we noticed that some of the veterans who had been housed in the big push in 2015 were starting to experience difficulties," Ms. Simons said. "We saw names starting to come back on the list. 'My gosh, how do we figure out how to prevent homelessness from someone who's recently been housed?' If we didn't have real-time data, we wouldn't have known so quickly. We started looking at other parts of our system to see if people started showing up at the food bank or a day shelter before they became homeless."

Sure enough, they did. That allowed them to notify case managers across agencies to do checkups to try to stem the inflow. "Often times those checkups were the difference between that person falling back into homelessness or getting additional assistance," Ms. Simons said. "Preventing homelessness costs so much less and is less devastating for the person."

Another region that has ended veterans' homelessness and is closing in on ending chronic homelessness is Montgomery County, Md. Homeless advocates there have also been using name lists not just to identify people who are experiencing chronic homelessness but also to catch people who are on the verge of becoming chronically homeless.

They have also taken the unusual step of identifying people in permanent supportive housing who don't want to be there and would be safe in regular housing, which is less intrusive and less expensive. To make the change happen, however, they needed to persuade the county housing authority to approve funding. "In order to make a successful budget ask, we needed to show that we had looked at programs and found efficiencies," said Nili Soni, the continuum-of-care coordinator for Montgomery County's Department of Health and Human Services. The person-level data helped them make the case.

All of these local successes get turned into mini case studies and are logged in Built for Zero's menu of strategies. So if a community partner wants to know how to do effective street outreach or improve housing retention or get more landlords to accept people who have been homeless as tenants, there's an inventory of proven ideas to draw from. For example, when Ms. Jaeger and Ms. Walker, in Rockford, were looking for ways to encourage landlords to accept higher-risk clients, they read about a community that had created a fund that could reduce the financial risk for l andlords. Then they got the local community foundation to do the same.

When it comes to ending homelessness, the natural questions are: "Isn't this about resources? Having enough housing dollars?" Mr. Maguire said. "Yes, more resources are definitely needed, especially in high-cost housing markets. But that won't get you all the way. Many of our communities already have the resources they need to end veteran homelessness, and the thing holding them back is the challenge of developing a smarter, faster, more coordinated housing system for that money to flow through."

"If you look at what led to the eradication of smallpox," Ms. Haggerty of Community Solutions said, "it was the construction of a surveillance system -- a culture of iteration, habits of ongoing problem solving, understanding that it needs to be a team, data to keep you connected and focused. We're finding that that's at the core of ending homelessness, too. There will always be people having housing emergencies, but we can shrink it to a totally manageable number so that homelessness becomes rare, brief when it happens, and doesn't recur."