The index found instances of slavery in every one of the countries looked at, with 45.8 million people enslaved overall—that’s up from 35.8 million in 2014. The forms of slavery included were sex trafficking, debt bondage, and forced labor. This estimate is more than double that of the United Nation’s International Labor Organization, though the U.N. doesn’t consider all forms of slavery.

The index looked at the prevalence of slavery, people’s vulnerability to it, and government efforts to fight against it. India and North Korea were deemed the worst offenders. India has the highest number of slaves overall, with 18.4 million, while North Korea had the highest concentration, with about 4.4% of its population enslaved. The five places in which government takes the least action are North Korea, Iran, Eritrea, Equatorial Guinea, and Hong Kong.

Walk Free founder Andrew Forrest told Reuters the large increase can be explained by better data collection, but he also worries that global displacement and migration make people more vulnerable to slavery.

The methodology included interviews with 42,000 people in 53 languages and 25 countries. While the index has been criticized by some, Forrest has challenged critics to find an alternative method. “Without measurement you don’t have effective management and there’s no way to lead the world away from slavery,” Forrest told Reuters. Efforts to combat slavery have been held back in the past because we don’t have hard data.

He said he wants this index to be a “call to action.” “This is not AIDS or malaria,” he told the news outlet. “We have caused slavery and because it’s a human condition we can fix it.”