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7 J. Human Trafficking 1 (2021)

handle is hein.journals/jhtrafk7 and id is 1 raw text is: JOURNAL OF HUMAN TRAFFICKING outledge 2021, VOL. 7, NO. 1, 1-13 u d https://doi.org/10.1080/23322705.2019.1660952 Taylor & Francis Group On the Unreliability of Multiple Systems Estimation for Estimating the Number of Potential Victims of Modern Slavery in the UK John Whiteheada, James Jacksona, Alex Balch', and Brian Francisa aDepartment of Mathematics and Statistics, Lancaster University, Lancaster, UK; 'Department of Politics, University of Liverpool, Liverpool, UK ABSTRACT KEYWORDS Accurate records of victims of modern slavery identified by various agencies Estimating numbers of allow investigators to compare different jurisdictions, track fluctuations in potential victims of modern prevalence over time and evaluate preventative interventions. As well as slavery; multiple systems enumerating those victims known to agencies, it would be desirable to know estimation; potential victims how many are working undetected under conditions of modern slavery and of modern slavery thus deduce the total number involved. To estimate the number of undetected potential victims of modern slavery in the UK, Bales, Hesketh and Silverman applied the method of Multiple Systems Estimation. Their approach involves fitting a statistical model to data listing victims detected by different agencies. In doing so, (a) they assume that various terms in the model are equal to 1, and (b) they only include terms not assumed to be 1 if they achieve statistical significance. In this paper, simulated datasets with known properties are used to show that if (a) is valid then (b) leads to substantial overstatement of the reliability of the estimates computed, and that if (a) is not valid then the estimation procedure is totally unsound. We conclude that Multiple Systems Estimation is not a suitable method for estimating numbers of potential victims of modern slavery. Introduction The most robust estimate to date of the scale of modern slavery in the UK was produced by the Home Office in 2014. The estimate suggested that there were between 10,000 and 13,000 potential victims of modern slavery in the UK in 2013 (Home Office, 2018, p. 4). This statement is made in the 2018 UK Annual Report on Modern Slavery, published in October of that year (Home Office, 2018). In the report, potential victims of modern slavery are taken to be people who have been identified and referred to as possibly having worked under conditions of modern slavery, and this includes both those whose victim status is eventually confirmed and those for whom it is not. For brevity, such people will be referred to as pvoms. In this paper we will show that the figures quoted above are unreliable, and that the method used to derive them is inappropriate. The range (10 000, 13 000) quoted in the Home Office report is drawn from a series of analyses presented by Bales, Hesketh, and Silverman (2015) (henceforth BHS) who deduced these figures using a method known as Multiple Systems Estimation (MSE). MSE is a generalization of the method of capture-recapture, used (for example) to estimate fish populations over a century ago (Goudie & Goudie, 2007; Manrique-Vallier, Price, & Gohdes, 2013). In such an application, a sample of fish from a stable population would be caught, counted, marked, and thrown back into the water. Later, a second sample would be caught, and the numbers of marked and unmarked fish would be CONTACT John Whitehead 0 j.whitehead@lancaster.ac.uk (D Department of Mathematics and Statistics, Lancaster University, Lancaster LA1 4YF, UK This article was originally published with errors, which have now been corrected in the online version. Please see Correction (http://dx.doi.org/10.1080/23322705.2020.1724650) © 2019 Taylor & Francis

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