Estimating the number of human trafficking victims in the United States is notoriously difficult.
"It is not that we haven't tried," said Amy Farrell, a criminologist at Northeastern University and one of the country's foremost experts on the topic.
In 2009 she looked at all the research done to that point and came up with a range of between 5,000 and 60,000 sex and labor trafficking victims. And she wrote a 95-page paper on why she couldn't narrow it down.
In the meantime, one of the most commonly cited figures that officials, advocates and journalists use when they try to measure child sex trafficking — 100,000 to 300,000 — is from a 2001 study that actually counted at-risk kids, not trafficked victims.
"When there have been attempts to estimate the size of human trafficking, it has been based on taking a very small unreliable number and making it a very large unreliable number," Farrell said.
It's important to get it right, she said.
"We see real people really being hurt and we're sure that there's quite a few of them," she said. "We're now getting attention to a problem that I see is very real, and I'm afraid it will all go away when someone over-inflates the number."
Barriers to research
Any illegal activity is hard to measure, but sex trafficking presents a unique challenge.
Under federal law, a victim of trafficking is anyone providing sex for money who is either under 18 or has been forced or deceived into doing it. But many of those people don't see themselves as victims. When it comes to adults, it's difficult to tell whether they're selling sex voluntarily or through coercion. And since commercial sex is hidden and stigmatized, its participants are hard to gather for a study.
"The traffickers themselves and the people who are providing commercial sex and being trafficked, they're not standing for censuses," said Michael Shively, a senior associate at Abt Associates, a research firm based in Cambridge, Mass.
The federal government has put significant funding into combating sex trafficking over the past decade and a half, but it's hard to know how much money is needed — or where it should go — without knowing the extent of the problem.
The FBI has been counting prostitution arrests for years, but only since 2013 has it maintained data on sex trafficking. In 2013, the FBI counted 13 offenses. In 2014, it counted 300. The FBI acknowledges those numbers are low, especially since it's just begun asking states to report.
Trying another tactic, the Bureau of Justice Statistics, part of the U.S. Department of Justice, teamed up with Northeastern University to collect information from federally funded anti-trafficking task forces. From January 2008 to June 2010, those task forces opened 2,065 investigations into suspected sex trafficking.
A limitation of both the FBI and the Justice numbers is that they measure only incidents that make it to the police.
One study that went beyond that was done in 2001 and suggested there were from 100,000 to 300,000 at-risk kids on the streets of North America, with higher estimates at over a million. The methods researchers used could have counted the same kids multiple times. Authors Richard Estes and Neil Weiner from the University of Pennsylvania explicitly cautioned against using the estimates incorrectly, but they frequently are.
Some researchers are going to the streets to get better numbers.
In 2008, Ric Curtis at John Jay College of Criminal Justice in New York used a complex method for studying hard-to-reach populations in New York City. Called respondent-driven sampling, it involves identifying a few "seeds" of the population and asking them to recruit others in their networks. He estimated that New York City had a hidden population of nearly 4,000 trafficked kids.
The Department of Justice commissioned Curtis and his colleague Anthony Marcus to replicate that work in Atlantic City, N.J., one of six cities the department wanted to study.
"They thought that we were going to finally find the missing victims that they've not been able to find," Curtis said.
"And we found more than they have," Marcus added. "But we didn't find that many."
Based on the six-city study, the Justice Department estimated that nationally there were probably between 8,915 and 10,507 children in the sex trade, with a maximum of about 21,000. About a third of the participants had some sort of "facilitator." Just under half of those were pimps who controlled by force, fraud or coercion.
That work, too, had shortcomings. The study authors acknowledged they likely missed a population of girls tightly controlled by pimps and kept indoors. But they said the study's importance is less about precise numbers than about obtaining a more complete sense of the trauma and economic barriers preventing victims from leaving the sex trade, whether they have pimps or not.
One value of the existing set of the FBI's prostitution arrest numbers over time is that it does reveal changes in police and prosecutor approaches, something that has been important as trafficking has moved to the forefront of concern. For example, for years, roughly two-thirds of prostitution arrests have been of women, but that is changing in some places.
In Arizona, for example, prostitution arrests went from 72 percent female in 2004, to 39 percent in 2014.
Although not all prostituted people are female and not all traffickers and sex buyers are male, a reversal that dramatic is a measure of a new approach by authorities.
"What we've really seen here in Arizona is a shift of culture," said John Raeder, director of violence prevention for the Governor's Office of Youth, Faith and Family.
The state is helping train officers to arrest sex buyers and to offer services to the sellers — whether they're trafficked or voluntarily engaging in prostitution.
"It's a difference without a distinction," said Raeder. "They're so interconnected that we don't ask law enforcement to make that determination. We ask that all law enforcement view these women as victims first."
'We know enough'
Researchers think the picture they have of trafficking will become clearer as years go by.
"I think in five to 10 years we are going to have a much better answer," Shively said. "A lot of the sort of squabbling and disagreeing and arguing is symptomatic of the fact that we haven't been trying to measure this right for all that long."
Meanwhile, researchers are uncovering new and practical information about who's being exploited — and how to help.
In the fall of 2014, the University of Minnesota Urban Research and Outreach Engagement Center released a study on the sex trafficking of minor girls in Minneapolis.
Principal researcher Lauren Martin and her colleagues went deep into the commercial sex market in the city. They scoured police reports and court records, analyzed years of media coverage and interviewed dozens of police and advocates.
They found that kids were recruited from bus stops, schools and even juvenile detention centers. They found that most of the victims were young girls of color. And they found that victims and traffickers came from the same low-income neighborhoods, while buyers were from just about everywhere.
Martin did not try to estimate the population, instead focusing on the intricacies of the market, from street prostitution up to private buying clubs.
"For whatever reason people really are fixated on counting," she said. "As if by counting victims we can put our arms around the whole problem and solve it."
Martin said she believes a national prevalence study can and should be done.
"Having answers to those questions is important for long-term decisions about resources," she said. "But ... we know enough to know what we need to be doing right now, too. So I wouldn't want us to wait around."