The 2018 edition builds on the Global Estimates of Modern Slavery, which estimated that 40.3 million people were in some form of slavery on any given day in 2016.
The Index provides rankings across three dimensions:
- Size of the problem: estimated prevalence in terms of percentage of population and absolute numbers (by country)
- Government response: How governments are tackling the problem
- Vulnerability: Factors that explain or predict prevalence
The Index targets private citizens, non-governmental organisations, businesses and public officials so that they can work to end modern slavery. All data involved are available for download from the website.
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The 2018 Global Slavery Index includes data on three key variables: the prevalence in each country, vulnerability and government responses. In 2018, the methodology underwent changes and significantly expanded its data sources. The methodology is detailed in the report.
In 2017, the inaugural Global Estimates of Modern Slavery were produced by the International Labour Organization (ILO) and the Walk Free Foundation in partnership with the International Organization for Migration. Acknowledged data gaps in earlier editions, including lack of data on forced sexual exploitation and children in modern slavery were addressed by adopting a combined approach when developing the estimates. This involved drawing on three sources of data:
- The survey program was expanded to include 54 surveys covering 48 countries. More than 71,000 people were interviewed and the surveyed countries represent over half of global population. It formed the most extensive survey program on modern slavery ever undertaken.
- Administrative data from the International Organization for Migration’s databases of assisted victims of trafficking
- Data derived from validated secondary sources and a systematic review of comments from the International Labour Organization supervisory bodies regarding ILO Conventions on Forced Labor.
The 2018 Global Slavery Index uses the data sources and regional and global estimates from the Global Estimates of Modern Slavery.
The regional estimates form the starting point for the 2018 national level estimates for 167 countries. Prevalence estimates from the 2018 Global Slavery Index were calculated according to the following process:
- Individual and country-level variables that have a significant relationship with forced labour or forced marriage at the individual level were identified. Data for this analysis were taken from Gallup World Poll (GWP) surveys conducted in 2014, 2015 and 2016.
- These risk factors were used to build a statistical model that best predicted occurrence of modern slavery at the individual level.
- Individual predictions were aggregated into risk scores at the country level. Whereas survey data on forced labour and forced marriage are not available for every country, a broader set of survey data covering variables such as age, gender, marital status and so on was available for 147 countries. Country risk scores were used to estimate country prevalence, based on the extent to which the country risk score deviated from the average regional risk scores.
- The number of victims was then estimated by applying the estimated prevalence to population data for each country. To this “base” estimate, an estimate of state-imposed forced labour was added to determine the final estimated prevalence of all forms of modern slavery.
The Global Slavery Index was criticized for its methodology employed to produce prevalence estimates for the 2013, 2014 and 2016 editions. The 2016 prevalence estimates were based on results of surveys in 25 countries through the Gallup World Poll. Its results were extrapolated to countries with an equivalent risk profile. Measurements of forced sexual exploitation and enslaved children were identified as critical data gaps to address in future estimations. The 2018 Index experienced substantial methodological improvements, including a significant increase in the number of survey data points, and substantial changes to the approach to estimating prevalence in countries without survey data.
Researchers Andrew Guth, Robyn Anderson, Kasey Kinnard and Hang Tran claimed that the 2014 Global Slavery Index's methods reveal weaknesses and raised questions about its replicability and validity. They stated that the use of its data can lead to inaccurate policy formulation and that the methods used in the Index are inadequate.
The 2014 Global Slavery Index assigned countries for which no data were available the same rate as surveyed countries that were judged to be similar. For example, prevalence rates for Britain were applied to Ireland and Iceland, and those for America to western European nations, including Germany. This extrapolation attracted criticism.
Scholar Anne Gallagher said the 2014 Global Slavery Index was based on flawed data. Gallagher writes "the basic unit of measurement of “modern slavery” is flawed: the definition is self-created and, bizarrely, changes from one year to the next."
Alexis A. Aronowitz called the Index to be "terribly flawed" and pointed out that:
"The index is based on mix of sources: population surveys in a few countries; fuzzy estimates by governmental agencies or NGOs; stories in the media; and local experts. For nations lacking any such source, the index creators engage in an "extrapolation" exercise -- they simply apply an estimate from one nation to "similar" nations lacking such estimates."
- "Global Estimates of Modern Slavery" (PDF).
- "Prevalence | Global Slavery Index". www.globalslaveryindex.org.
- "Government Response | Global Slavery Index". www.globalslaveryindex.org.
- "Vulnerability | Global Slavery Index". www.globalslaveryindex.org.
- "Downloads | Global Slavery Index". www.globalslaveryindex.org.
- "Global Estimates of Modern Slavery: Forced Labour and Forced Marriage". 2017-09-19. Cite journal requires
- Inc., Gallup,. "What If You Were the Leading Source?". Gallup.com. Retrieved 2018-10-12.CS1 maint: extra punctuation (link)
- Andrew Guth, Robyn Anderson, Kasey Kinnard and Hang Tran, Proper Methodology and Methods of Collecting and Analyzing Slavery Data: An Examination of the Global Slavery Index, in Social Inclusion (open access journal), Vol. 2, No 4 (2014), pp. 14-22, article posted on the Cogitatio website on 17 November 2014: "The Global Slavery Index aims to, among other objectives, recognize the forms, size, and scope of slavery worldwide as well as the strengths and weaknesses of individual countries. An analysis of the Index’s methods exposes significant and critical weaknesses and raises questions into its replicability and validity" (summary of the article) - "The formation and implementation of sound policy is not possible without sound data. The methodology and methods used in the Index are currently inadequate and therefore the Index cannot be validated or replicated. Furthermore, the publicity given to the Index is leading to the use of this poor data not only by popular culture and reputable magazines and news organizations [...], but also by academic journals and high level policy makers [...], which can lead to inaccurate policy formulation and a compounding of harm [...]" (p. 19).
- "Economist Online: Performance indices - International comparisons are popular, influential—and sometimes flawed". The Economist. Retrieved 2014-11-16.
- Gallagher, Anne (November 28, 2014). "The global slavery index is based on flawed data – why does no one say so? | Anne Gallagher" – via www.theguardian.com.
- Human Trafficking: A Reference Handbook, Alexis A. Aronowitz, ABC-CLIO, 2017, p. 172