More than 29 million people are living in slavery, according to the first index to attempt to measure the scale of modern-day slavery on a country-by-country basis.

The index, published by the Walk Free Foundation on Thursday, ranks 162 countries and identifies risk factors for enslavement and the government responses.

The research found that around 10 countries hold about 70% of the world's slaves. India has the highest number of people enslaved in absolute terms, approximately 14 million, almost half the total worldwide. China has 2.9 million enslaved and Pakistan is third, with an estimated 2 million.

Lack of data has long frustrated anti-slavery campaigners, who argue that without knowing who is being exploited and where, it is impossible to properly tackle the problem.

Nick Grono, CEO of the Walk Free Foundation, said: "There is no index out there, and it's critical to measuring the scale of the problem on a country-by-country basis because that informs policy responses. Measuring a hidden crime is very challenging, but there are efforts to measure domestic abuse and drug trafficking. A lot of it boils down to taking the best data on reported issues and then looking at the scale of the unreported or 'dark' problems."

The index builds on statistics compiled by the International Labour Organisation, which estimates 21 million people worldwide are in forced labour.

"It's very similar to our method," said Grono, "going country by country, taking the reported data and then estimating the 'dark' figures. They haven't published country-by-country data."

Kevin Bales, the lead academic behind the index, has written extensively about the issue of modern-day slavery. He said the index is the culmination of many years work looking at how to make campaigns more effective. "I've been anxious to do this for getting on 15 years. 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."

For Bales, the data is the first step in being able to tackle exploitation, wherever it exists. "Let's really focus on those countries, all of those countries are going to get some very devoted attention," he said.

Grono acknowledged the controversial nature of any attempt to compile hard facts on slavery and human trafficking, which are by their nature conducted in the shadows.

"We recognise the data is not that strong; we want to be open about this. If a government says they don't agree [with the data], we will say great, let's work with a national statistics office to do a study across the country to try and analyse the scale of the problem."

As well as a tool to hold governments to account, part of the challenge for the index authors will be to persuade researchers that the data is useful.

Not everyone is convinced. Bridget Anderson, deputy director of the Centre on Migration, Policy and Society at the University of Oxford, who has researched and written about human trafficking, said any attempt to gather "unjust situations" across the planet and label them as "slavery" is already getting off on the wrong foot.

"I wouldn't find it useful. You have a definitional problem, everything depends on the definition and if you use tricky words like 'forced', you are already straying into difficult territory," she said.

"Say with sex trafficking: if you are dealing with people who have very constrained choices, and you are so horrified with the choices, you say you are not allowed to make that choice, it's too terrible for me on my nice sofa to tolerate. Is it right that you shut that choice down? It's unjust that someone could be in such a situation where poor options are seen as great opportunities."

"Slavery is such an emotive term. It has an incredible emotive power, which is what a lot of campaigners use and draw on productively, but then people engage with their emotions but don't think about it politically. It becomes de-politicised."

But Bales is already working on next year's index, in which he plans to include more research on the ground in the form of random sample surveys. He is passionate about his cause and believes the accurate labelling and tracking of the "disease" of slavery is the first step in beating it.

"It's a lot like epidemiology," he said, "we have to get in there and vaccinate – but first we have to figure out where the disease is, what are the risk factors. So in the same way that you do that with an epidemic, it's not always in the same place, there are different supporting factors … It's been a lot of work but it's been very exciting."