"Improving the Statistics and Prevalence Data on Human Trafficking in the United States"
This commentary from the Trafficking Research Project is a helpful introduction to the methodological issues involved in producing reliable estimates of human trafficking.
Crime in the United States
Since 2013, the Federal Bureau of Investigation has recorded trafficking-related offenses (forced labor and sex trafficking) via the Uniform Crime Reports program for collection of crime data. Note that the collection of information on such offenses is uneven across the states.
Economics of Human Trafficking
The Economics of Human Trafficking project at the University of Marburg in Germany is devoted to measuring anti-trafficking policies of individual states, specifically by examining policies with regard to prosecution of trafficking, protection of its victims, and prevention of trafficking. The data are available as the 3P Anti-Trafficking Index covering nearly 200 countries. You can read more about the project's work at http://www.uni-marburg.de/fb02/empinsti/research/human_trafficking. Earlier iterations of the 3P index data are available at http://www.uni-goettingen.de/en/anti-trafficking-policy-index/204468.html, http://www.uni-heidelberg.de/fakultaeten/wiso/awi/professuren/intwipol/datasets_en.html, and http://humantraffickingindicators.blogspot.com/p/about.html.
European Commission Migration and Demography Data Catalogue
The European Commission has a very useful catalogue on data sources related to migration and population movement. You can search for terms or filter the full list of resources by either broad topics (e.g. aslyum and refugees, forced trafficking) or by specific terms/tags (e.g. "labour," "smuggling").
The Global Dataset is a project by the Counter-Trafficking Data Collaborative and consists of anonymized data on individual victims of trafficking. The data include variables such as country of citizenship, country of exploitation, nature of coercion, nature of exploitation, and gender of the victim. Researchers interested in non-anonymized data need to contact the CTDC directly.
Human Trafficking Indicators
The Human Trafficking Indicators draw from the U.S. State Department's annual Trafficking in Persons Reports and code individual countries on whether they are sources, transit points, or destinations for trafficking flows across borders, whether trafficking takes place within their borders, the types of trafficking (e.g. labour, prostitution) that take place, and their efforts to prosecute trafficking cases and protect trafficking victims. The data were collected as part of the Human Trafficking Indicators project.
International Labour Organization (ILO)
The ILO is a United Nations agency that adovocates on behalf of and conducts research on matters related to labour rights and labour standards. The organization produces an extensive collection of statistics and databases covering various topics, including child labour and labour migration. The ILO's topical areas of interest also include work on involuntary migration and labour.
United Nations Office on Drugs and Crime (UNODC)
The UNODC provides various resources about human trafficking and migrant smuggling, including a portal for legislation and cases related to trafficking. UNODC also produces an annual report on trafficking in persons.
United Nations Surveys on Crime Trends and the Operations of Criminal Justice Systems (UN-CTS)
The UNODC also conducts a recurring survey on crime and criminal justice statistics. Since the mid-2000s, the survey has included questions on human-trafficking offenses. Results from the survey are distributed across various locations, including https://www.unodc.org/unodc/en/data-and-analysis/statistics/crime.html and https://data.unodc.org/. Results from the survey are also available via the ICPSR.