Nearly 30 million people across the world are currently living in slavery, according to a report published Thursday. The inaugural Global Slavery Index, compiled by the Walk Free Foundation, estimates the prevalence of slavery in 162 countries, based on both internal research and data from UNICEF and the US State Department's Trafficking in Persons report. According to the report (PDF), slavery is most prevalent as a percentage of the population in Mauritania, Haiti, and Pakistan. India, China, and Pakistan are among the countries where slavery is most prevalent in absolute terms; together with Nigeria, Ethiopia, and five other countries, they account for more than 75 percent of the world's enslaved population.

Walk Free was founded last year by Andrew Forrest, an Australian billionaire who made his fortune in the mining industry. Its index has received endorsements from Microsoft co-founder Bill Gates, former US Secretary of State Hillary Clinton, and former UK Prime Minister Tony Blair.

"I urge leaders around the world to view this index as a call to action, and to stay focused on the work of responding to this crime," Clinton said in a statement provided to the Associated Press.

The index defines "slavery" as "the condition of treating another person as if they were property — something to be bought, sold, traded or even destroyed." It also accounts for human trafficking and forced labor — defined as "work taken without consent, by threats or coercion — grouping all three indicators under the umbrella term "modern slavery".

Reliable data on slavery has been hard to come by, due in large part to inconsistent definitions and a lack of coordination among international organizations. Walk Free aims to overcome these challenges by evaluating and combining existing data with statistical extrapolations based on a wide array of sources.

"I've been anxious to do this for getting on 15 years," Kevin Beales, a professor at the University of Hull and the study's lead author, told the Guardian. "It's a hidden crime, but there is a lot of collection of secondary source of information, NGO records, police records. We basically ransacked the planet to get those."

"The global stats on slavery are still very rough estimates and it is important to describe and quantify the problem in order to tackle it effectively," Aidan McQuade, director of UK-based charity Anti-Slavery International, said in an email to The Verge. "That's why the Slavery Index is important, even though it is not perfect. It is a good starting point of conversation and engaging decision makers to tackle this issue."

Some of Walk Free's findings are rather startling. In Mauritania, between 140,000 and 160,000 of the country's 3.8 million inhabitants are believed to be living in modern slavery. In Haiti, about one in ten children are exploited under a child labor system. An estimated 14 million people are enslaved in India, making it by far the biggest offender in absolute terms (neighboring Pakistan came in second, with 3 million). Walk Free blamed much of India's problems on an inefficient legal system and a tradition of "servile marriage."

The worst offenders are largely low income or developing countries, though rich nations aren't exempt, either. In the US, an estimated 59,000 people are being exploited for labor, largely due to high demand for human trafficking. "The relative wealth of Canada and the United States, their demand for cheap labor and relatively porous land borders, makes them prime destinations for human trafficking," the report reads.

The issue has drawn renewed focus in recent months, following allegations that Qatar has been exploiting migrant labor to build new stadiums and infrastructure ahead of the 2022 World Cup. Some activists have estimated that construction there could leave as many as 4,000 migrants dead by the time the event begins in nine years.

Walk Free acknowledges the challenges in trying to quantify an inherently underground market, though the organization hopes that its work will spur global leaders to take greater action.

In a video statement released Thursday, Gates compared Walk Free's index to the Millennium Development Goals that the United Nations  "The index can be an equally important tool to help governments, NGOs, and business take stock and take action against this terrible problem," Gates said.