The TLDR is: they surveyed broadly representative demographics of nine countries on whether any family members had ever been forced to work, and kept in that position by coercion.
They also used some secondary sources for ten other countries.
They extrapolated the figures for the rest of the 167 countries based on averages of countries considered to have similar characteristics, with some largely subjective country-specific adjustments.
As an example, the score for China, which they note has few confirmed reports of widespread slavery was
(average proportion of slaves in Japan, South Korea and Taiwan).9 + (average proportion of Qatar, Malaysia, and Saudi Arabia).1
Worthy as the objective behind the research may be, it's difficult to understate how speculative these estimates and the percentages reported to a ludicrous number of decimal places actually are.
The TLDR is: they surveyed broadly representative demographics of nine countries on whether any family members had ever been forced to work, and kept in that position by coercion. They also used some secondary sources for ten other countries.
They extrapolated the figures for the rest of the 167 countries based on averages of countries considered to have similar characteristics, with some largely subjective country-specific adjustments.
As an example, the score for China, which they note has few confirmed reports of widespread slavery was (average proportion of slaves in Japan, South Korea and Taiwan).9 + (average proportion of Qatar, Malaysia, and Saudi Arabia).1
Worthy as the objective behind the research may be, it's difficult to understate how speculative these estimates and the percentages reported to a ludicrous number of decimal places actually are.