The Global Slavery Index is based on state of the art research methodology that has been developed with the assistance of an independent Expert Working Group, comprised of world leading experts. The methodology has also been subjected to independent external review. This estimate is based on data from nationally representative, random sample surveys conducted in 25 countries. All surveys were conducted face-to-face in key local languages using a standardised instrument. Collectively, these surveys represent 44 percent of the global population. The results of these surveys have been extrapolated to countries with an equivalent risk profile.