Steve Williams is dogged — so dogged that he got bit by an actual dog.

The dog attack happened while Williams was out doing his job: tracking down renters in the poorest parts of Milwaukee and trying to ask them questions. To reach some people, he’d call or return to their residence 10, 20 or 30 times. While pursuing another interview — knocking on doors with a clipboard — he was mugged, he said, but lost only $17. Collecting data can be dangerous. “I sort of see it as part of the job,” Williams said.

Williams took those risks to fill a big gap in our understanding of a crucial issue: eviction. Millions of Americans are forced to leave their homes every year, and experts see housing instability as a major contributor to a host of other problems that poor households face. But getting more precise than “millions” is impossible because of a lack of good data. The federal government does a poor job of tracking evictions, and the sources that do exist, such as court records, are incomplete and lack detail.

But that may be starting to change. The survey that Williams was part of, the Milwaukee Area Renters Study (MARS), may be the first rigorous, detailed look at eviction in a major city. Interviewers like Williams spoke to about 1,100 Milwaukee-area tenants between 2009 and 2011, asking them a battery of questions on their housing history. The survey has already fundamentally changed researchers’ understanding of eviction, revealing the problem to be far larger than previously understood. Now the survey is going national: The Census Bureau recently agreed to add some of the MARS questions to its massive, biennial housing survey.

MARS was the brainchild of Matthew Desmond, a Harvard sociologist whose recent book, “Evicted: Poverty and Profit in the American City,” brought national attention to evictions. The book chronicles the lives of several poor families living in a variety of housing situations, such as crime-ridden inner-city neighborhoods and blighted trailer parks. To supplement his on-the-ground work in these communities, Desmond went searching for data. First, he looked at court records to gather eviction statistics, but there were lots of questions that those records couldn’t answer, such as what are the demographics of people facing eviction. Then he looked at the academic literature, but that search “came up empty,” Desmond said. That’s when he reached out to the University of Wisconsin Survey Center, an academic research organization that specializes in reaching understudied groups: kids in foster care, welfare recipients, the homeless.

Conducting good survey research is hard. Conducting good survey research on people with low incomes — who tend to be transient, hard to reach and often hesitant to greet strangers knocking on their doors — is even harder. Desmond’s survey targeted people who are particularly tough to reach: low-income renters, who move more often than the general population. “The people who are easy to get in touch with aren’t the same as those that are hard to reach,” said John Stevenson, associate director at the survey center.

That’s where interviewers like Williams come in. The survey center runs a training program before sending staff into the field. “The hardest part is getting interviewers to keep going back,” said Kerryann DiLoreto, a senior project director for MARS. “It’s that fine line between not wanting people to think we’re stalking them and being very persistent.”

Interviewers were taught to time their arrival to catch tenants in the driveway, before they got behind closed doors. And to let residents know they were coming, the center sent out mailings, with a dollar or two inside so tenants would look at them before throwing them out. To get tenants’ attention, teams sometimes dropped goody bags containing a candy bar and notice outside their door.

“Interviewers have to be trained in presenting themselves quickly,” DiLoreto said. Some tenants rejected interviews if they suspected the interviewer of being a representative of the landlord, an agent from child protection services or a police officer. But through sheer persistence, the MARS project ended up with a response rate of over 80 percent, a remarkable figure for such a survey. “Great interviewers — we ride on their shoulders,” Stevenson said.

It wasn’t just the survey process that was tailored to the target population. The questions themselves were, too. Questionnaires used to interview poor people — what sociologists call “material hardship surveys” — tend to ask something like: “Have you ever been evicted?”

“That seems like a valid question,” Desmond said. “But when you spend time with low-income tenants, you realize that’s incomplete.” That’s because there are formal and informal evictions.

A typical image of an eviction may be when a sheriff with a court order and some tough guys move all a renter’s stuff to the sidewalk. But these formal evictions, Desmond found, were comparatively uncommon, making up only 24 percent of all forced moves, according to the MARS data.

Informal evictions were twice as common (48 percent of all forced moves). In these off-the-books evictions, a landlord might, for example, give a tenant $200 to move out by Thursday. Or they might take the door off. Regardless, it happens without a legal paper trail. (To round out the other reasons, the MARS survey found that about 23 percent of forced moves were because of landlord foreclosure and 5 percent because of a building condemnation.)

No matter the reason, the MARS researchers found that when people were forced to move, they often didn’t see it as an eviction. So instead of just asking, “Have you ever been evicted?” the MARS survey posed a roster of questions about a tenant’s housing history — when and where they had lived and why they left. This “moving module” was the centerpiece of the MARS study. By asking more than 250 questions, interviewers like Williams gathered data on every place a respondent lived for at least 30 days over the previous two years. Small wording details made a big difference. Rather than “Where do you live?” people were asked, “Where do you spend most nights?”

“When you say, ‘Where do you live?’ they’re thinking of something quite formal — like where they’re getting their mail or the address they’re registered for government benefits,” DiLoreto said. Where people live and whether they’ve been evicted are questions that aren’t as simple as they might seem.

Armed with arguably the most comprehensive data set on eviction ever collected, the MARS study produced a shocking finding: In the two years before being surveyed, more than 1 in 8 Milwaukee renters were forced to move, whether because of a formal or an informal eviction, foreclosure or condemnation. Also, Desmond’s follow-up research using MARS data has found a strong connection between eviction and subsequent residential instability, even after factoring in the tenant’s income and race. Eviction is linked to substandard housing conditions. And eviction also has serious negative health consequences, particularly for children.

Eviction also has a racial and gender bias: “Among renters, over one in five black women report having been evicted sometime in their adult life,” Desmond has found. “If incarceration had come to define the lives of men from impoverished black neighborhoods, eviction was shaping the lives of women,” Desmond wrote in his book. “Poor black men were locked up. Poor black women were locked out.”

The MARS survey has drawn so much attention for its innovative questions that the federal government is adopting it into the American Housing Survey, a massive biennial survey on homes, housing costs and related subjects. The survey didn’t ask about eviction at all until 2005; now, it has one question on the subject.

“It’s fair to say we have not collected good data in the past on evictions,” said Shawn Bucholtz, the director of the division that runs the biennial survey at the Department of Housing and Urban Development, which administers the survey in conjunction with the Census Bureau. The results of the 2015 survey — which will be released next month — won’t contain much data on evictions, but starting with the 2017 survey, things will change. That’s because HUD is adopting the MARS questions “pretty much verbatim,” Bucholtz said. “It was a pretty easy decision to make.”

By pioneering how to ask about eviction, and showcasing its prevalence, the MARS study has spurred policymakers to take it seriously. “I’m unaware, at a federal level, of any other deep-dive into eviction,” Bucholtz said. “It’s just not a question that pops up on our major household surveys.”

Evictions, like other subjects related to poverty, have historically been understudied, in part because surveys like MARS are expensive and time-consuming. But Stevenson said that avoiding this kind of research is a false savings. “Not studying them is not making them go away,” Stevenson said. “They don’t go away; they become bigger, more expensive problems.”

Read more: Why So Many Poor Americans Don’t Get Help Paying For Housing