But how have Bales and his colleagues arrived at the figures?
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The Global Slavery Index in common with the ILO uses what are called secondary sources. These can be government figures, NGO research and reports in the media. But Kevin Bales says another key tool is the random sample survey where researchers carry out interviews with real people to collect data on slavery.
They currently have fewer than a dozen of these country surveys. They then mathematically extrapolate this data to other countries where they have no survey data.
"I believe it is reasonable," says Bales. "We called in a very large number of statisticians who independently assessed our methodology before we went public and they also agree that it was acceptable."
However he does admit that the interviewing people to collect statistics is a method that risks double counting when people are enslaved in a different country to the one they are from.
So can we trust these figures? Or is there too much guess work involved to take them seriously?
"We may be completely off the rails, I can't deny that," says Bales. "When you are confronting a crisis and you are coming up with the best numbers you can, of course it's right to put that number out there."