Fighting slavery through statistics: 5 promising methods to estimate prevalence

A t-shirt with a message against human trafficking worn during an anti-human trafficking event in Laos. Photo by: Thomas Wanhoff / CC BY-SA

It is much easier to protect what one can count and identify.

Human trafficking prevention and intervention efforts can become more effective and targeted with more accurate information on how many victims exist and where they are likely to be found.

Measuring prevalence in modern slavery has historically presented several challenges. Definitional disagreements, lack of national coordination between all anti-trafficking stakeholders and their data collection systems, and limited funding have played roles in the difficulties of obtaining a widely accepted and methodologically rigorous national prevalence estimate for human trafficking in the United States.

As one of the first countries in the world to pass a comprehensive national anti-human trafficking law, and one of the only countries in the world to publish its annual Trafficking in Persons Report through the State Department, the U.S. has the opportunity to maintain its leadership and continue its progress on this critical human rights issue. But there needs to be a more focused and customized effort to estimate human trafficking in a country as populous, diverse and critically nuanced as the U.S.

Human trafficking prevalence measurement in developed countries is exceptionally difficult due to effective rule of law and generally low levels of extreme vulnerability, rendering detection unlikely within standard survey sample sizes.

However, other developed countries such as the U.K. and the Netherlands have managed to utilize alternative methods to obtain national prevalence estimates. Unfortunately, if we only continue to study human trafficking in developing countries — where it may be traditionally easy to find through survey methods — we will be mischaracterizing the established and dominant role of stronger market economies and developed countries in this issue.

To better advocate for committed and sustained funding for anti-trafficking efforts in the United States, and to develop benchmarks and baselines for future research and progress measurement, it is critical to come together as a field and engage in collaborative efforts to determine national prevalence estimates.

While there have been attempts to do this in the past, concerns have been raised that such studies focused only on specific populations, such as minors or foreign nationals, and on specific industries, such as commercial sexual exploitation of minors or agricultural labor. Some may argue that these industries are easier to identify or target, but that there are substantial issues related to networks that are more difficult to identify, such as domestic servitude and traveling sales crews.

However, there are potential solutions to this. The following five methods are among the most promising opportunities to develop a national human trafficking prevalence estimate in the short- to medium-term future.

1. Multiple systems estimation was successfully conducted in the United Kingdom in 2014 and in the Netherlands in 2016. In this method, three or four concurrent lists of identifiable victims are analyzed for apparent overlap — incidences where a specific victim was named on one or more lists in the same time period. This helps statisticians to estimate how likely it is that the known list of victims collected by these groups are identifying the same victims and reveals who they may be missing.

This method is generally considered to be the most reliable and is ideally suited for developed countries, where generally lower vulnerability conditions may result in overall lower prevalence rates, while stronger law enforcement and rule of law makes human trafficking more difficult to detect.

Davina P. Durgana.

2. Respondent-driven sampling was originally introduced by Douglas Heckathorn to provide sufficient statistical parameters to chain-referral methods to determine statistically valid indicators and to counteract issues.

Originally, respondent-driven sampling — a process in which initial survey respondents are identified and then asked to identify other respondents who share similar characteristics in an attempt to locate difficult to find populations — was challenged by a lack of statistical parameters that would allow statisticians to infer the total population from this non-random sample. However, Heckathorn’s contributions allowed researchers to begin to use this effective method of identifying respondents, as well as to statistically infer information about the total population.  

Sheldon Zhang has championed this work in the anti-trafficking field among migrant workers in San Diego, and North Carolina has improved upon the referral methods to further reduce the biases that have traditionally reduced the statistical validity of such approaches. While promising, this type of chain-referral approach also requires incentives for participation and may be difficult and costly to replicate on a national scale.

3. Online-based surveys of U.S. stakeholders is an important and relatively low-cost possible method of obtaining more information on human trafficking prevalence data and related issues, but would be difficult to use for statistical inference. There have been several promising efforts, such as the pilot online survey of United Against Slavery. They now plan to begin integrating questions related to human trafficking data and prevalence into their survey design.

4. National aggregation of statewide prevalence estimates is another possible method. There is a recent study from the University of Texas in Austin that used available administrative data and targeted qualitative interviews in Houston to estimate that 313,000 people are enslaved in Texas. This is a promising step for a large and populous state, but for this method to gain traction, other states will have to follow suit with their own statewide estimates.

5. Gallup public opinion surveys on human trafficking awareness and willingness to report to hotlines and/or law enforcement is a potential method that has already yielded some compelling preliminary results.

In early 2016, Gallup World Poll included a set of questions related to human trafficking awareness and the general public’s willingness to report human trafficking in the Gallup Daily Poll for the U.S. This information is useful to the field due to the valuable, publicly accessible, and anonymous case and call information that is published online on calls to the National Human Trafficking Resource Center and Hotline.

While the survey in 2016 indicated that many respondents would report a crime of human trafficking to law enforcement directly, there were an encouraging number of respondents who were both aware of the hotline and indicated that they would report crimes there.

 “As one of the first countries in the world to pass a comprehensive national anti-human trafficking law, and one of the only countries in the world to publish its annual Trafficking in Persons Report through the Department of State, the U.S. has the opportunity to maintain its leadership and continue its progress on this critical human rights issue.”

— Davina P. Durgana

This method can be further developed alongside a more robust vulnerability model to try to determine the statistical validity of using the publicly available information on calls to the National Human Trafficking Hotline as an approximation of the incidence of human trafficking in the U.S.

These efforts should also reflect the best practices in applied research and replication standards, including transparent and reproducible methods as well as the development of theoretically grounded models. The development of a theoretically grounded model in particular is necessary to advance our understanding of vulnerabilities to human trafficking in the United States and around the world. To date, there has been moderate success to this effect by employing the U.N.-derived human security framework to this effort.

However, given the rapid changes in public awareness of human trafficking and diversity of thought on vulnerability modeling to human trafficking, this approach will require much more academic and practitioner collaboration and discussion, as well as possibly an updated survey. The U.S. government is making significant progress in evaluating the feasibility of MSE for the U.S. context and human trafficking data, and many stakeholder groups and nonprofits such as the McCain Institute United Against Slavery continue to address these issues.

Further engagement on these issues and methods are necessary to the field’s overall success in obtaining a reliable national prevalence estimate. Anyone interested in contributing to these efforts is encouraged to consider how their talents may be best used in assisting these innovative and multidisciplinary approaches to measure slavery and should share their experiences with best practices from other related development issues. Many organizations such as the Walk Free Foundation’s Global Slavery Index Report team — which includes the author — are open to collaborating with a wide array of international development professionals on this work and welcome your contact and participation.

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About the author

  • Davina P. Durgana

    Davina P. Durgana, Ph.D., is senior researcher and statistician on the Global Slavery Index of the Walk Free Foundation and also teaches at SIT Grad Institute. She was recently named the 2016 Statistical Advocate of the Year by the American Statistical Association and as a Forbes Top 30 Under 30 in Science for 2017 for her work on statistical modeling, human security theory and human trafficking. She focuses on applying analytical models to understanding vulnerability, risk and prevalence on the issue of human trafficking domestically and internationally.

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