UNODC’S Vision For Measurement of Trafficking in Persons

30 July 2018
Research Innovation

Raggie Johansen  | Research Officer, UN Office on Drugs and Crime

For many years, the international community has sought a way to accurately gauge the number of victims of trafficking in persons. The measurement of crime in general, and trafficking in persons specifically, is challenging because much of it remains undetected and unreported. While UNODC and others have considerable data on the number of detected cases of trafficking, the “dark figure” remains unknown.

The quest for sound magnitude data was boosted by the adoption of the Sustainable Development Goals (SDG) in 2015. Three SDG targets explicitly refer to trafficking in persons, and SDG Indicator 16.2.2 calls for reporting on the number of victims of human trafficking per 100,000 population, disaggregated by sex, age and form of exploitation.


Housing along waterways in Ho Chi Minh, Vietnam. Unsplash/Tony Lam Hoang

UNODC has been collecting data on detected victims of trafficking in persons from national criminal justice and other official sources since 2003. The regular data collection that forms the backbone of the biennial Global Report on Trafficking in Persons includes national-level data from more than 130 countries across all regions of the world. This data can give invaluable information about patterns, trends and flows of trafficking at the national, regional and global levels—but detected cases cannot be used to measure prevalence since so many trafficking cases remain undetected.

Recent methodological advances may have put an overall victim estimate within reach, however. One promising research approach—Multiple Systems Estimation (MSE)—could help bridge the current knowledge gaps by enabling countries with strong victim detection and recording practices to generate a solid national victim estimate based on reliable sources and methodologies. For countries that are not in a position to detect or properly record sufficient numbers of trafficking victims, a better option might be to carry out surveys that have been tailored to reach this statistically rare and often hidden population.

Many countries—particularly those that are typical destinations for trafficking victims—would meet the data and capacity requirements for carrying out MSE. In brief, MSE is an elaboration of the capture-recapture methodology, tailored for application on multiple lists of detected victims of trafficking in persons. The victim lists can come from a range of sources, such as law enforcement, the judiciary, NGOs, shelters, labour inspectors, immigration services or local authorities. MSE works by determining the overlaps of unique victims who appear on the different lists, and based on these overlaps, calculating how many victims there are likely to be in total.

Capture-recapture methodologies have been used in other contexts involving “hidden populations” in the past, for example, to estimate the number of casualties in armed conflict and numbers of people who inject drugs. MSE relaxes the condition of independence of the victim list, which is required to implement capture-recapture, but it does require that at least three different victim lists are available. Through MSE studies, the “hidden figure” of trafficking victims can be revealed, which means that an overall victim estimate can be produced. Moreover, if the available data is sufficiently detailed, MSE can generate estimates disaggregated by sex, age and form of exploitation, as called for in Indicator 16.2.2.

To date, MSE studies on the magnitude of trafficking have been carried out in five countries. The first was the United Kingdom, where an estimate of the number of victims was published in 2014. In 2016, UNODC and the Dutch National Rapporteur on Trafficking in Human Beings carried out an MSE on the number of presumed trafficking victims in the Netherlands, and in 2017, they expanded and refined the previous year’s study by including multiple years and disaggregated results by sex, age, form of exploitation and whether or not victims are Dutch citizens.

Through MSE studies, the “hidden figure” of trafficking victims can be revealed, which means that an overall victim estimate can be produced.

UNODC has also carried out MSE studies with the relevant national authorities of three other European countries in collaboration with the Walk Free Foundation. The results of these studies will be published later this year and will include results disaggregated by sex, age and form of exploitation. UNODC plans to carry out MSE studies in seven more countries in Europe, Asia and Latin America over the next three years. These studies will require close collaboration between UNODC, with its methodological and statistical expertise gained from multiple past studies, and the relevant national authorities, with their intimate knowledge of the country’s trafficking situation and the particularities of the data contained in the victim lists.

As noted above, to carry out studies with the MSE methodology requires the existence of multiple lists of detected victims. In many countries, particularly more typical origin countries for transnational flows, such lists of detected victims do not exist, or are not sufficiently comprehensive or comparable. Such countries are good candidates for targeted victimization surveys using the network scale-up method, which relies on conducting a survey of the general population, in which questions are asked about the number of individuals of interest in the personal network of the respondent and a specific set of questions are devised to estimate the size of the respondent’s network. Carrying out surveys by using this methodology is time- and resource-intensive, but imposes fewer requirements on participating countries. It is UNODC’s intention to seek resources to pilot test network scale-up-based surveys to uncover the real extent of trafficking in persons in some key origin countries.

Provided that both methodologies withstand rigorous testing and assessment in the pilot countries, UNODC plans to develop measurement standards that can be used by the international community that will include detailed methodological guidelines to assist Member States eager to carry out their own prevalence estimates. Once a critical mass of national estimates have been completed—whether by MSE or through surveys—regional and eventually also global estimates of the number of trafficking victims can be produced.

Raggie Johansen is a Research Officer at the Crime Research Section of the UN Office on Drugs and Crime.

This article has been prepared by Raggie Johansen as a contribution to Delta 8.7. As provided for in the Terms and Conditions of Use of Delta 8.7, the opinions expressed in this article are those of the author and do not necessarily reflect those of UNU or its partners.

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