Amongst other things, the SDGs present very real challenges for measurement. What is the prevalence of modern slavery right now? And what constitutes an effective measure to eradicate forced labour or human trafficking?...Ideally data for measurement of the SDGs should come from actual counts – but in the context of modern slavery, as with many serious crimes, this is unrealistic. Whether in the form of forced labour on Thai fishing vessels or forced marriage in South Asia, modern slavery tends to be deeply hidden and rarely reported. Official statistics do not reflect the actual size of the problem....In the 2014 Global Slavery Index, we generated national estimates of modern slavery for 167 countries through extrapolation. That is, we used a statistical method, at the core of which was hard data on prevalence for a limited number of countries, plus data on factors that are relevant to vulnerability to enslavement.