LONDON: More and better data is needed to track progress in the global drive to eradicate modern slavery and human trafficking as many victims - including people trafficked for their organs and child soldiers - are going uncounted, leading anti-slavery groups say.

About 40 million people were trapped as slaves last year - mostly women and girls - in forced labour and forced marriages, according to the first joint effort by key rights groups to count the number of victims worldwide, published last month.

The UN's International Labour Organization (ILO) and the Walk Free Foundation collaborated on the estimate - which they said was very conservative - having previously used different data, definitions and methodologies to reach separate figures.

The groups last week published the methodology, a month after the estimate was released, referring to limitations and gaps in the data, such as the omission of child soldiers and a lack of surveys for conflict-hit countries and the Gulf region.

But critics say the estimate cannot be used to guide governments, shape specific interventions or monitor successes or setbacks in the world's fight to end slavery by 2030 - one of the UN Sustainable Development Goals (SDGs) adopted in 2015.

"While the estimate may be useful for advocacy purposes, it is not evidence," said lawyer and slavery expert Anne Gallagher.

"There is currently no way to use this information in a manner that would allow us to know that what we are doing is making a difference," she told the Thomson Reuters Foundation.

The ILO and Walk Free said they shared the frustration of the anti-slavery movement about the fact the estimate cannot be compared with previous figures, and that the data is not comprehensive enough to track progress in tackling the crime.

"But we have developed a method which is sustainable, and allows us to build more and more reliable estimates year-on-year," said ILO senior statistician, Michaelle De Cock. "Once we have more and better data, we'll be able to measure trends."

BASELINE

Among the estimated 40.3 million people living as slaves last year, 24.9 million were forced to work in factories, on building sites, farms and fishing boats, and as domestic or sex workers, while 15.4 million were trapped in forced marriages.

This compares with Walk Free's 2016 number of 45.8 million slaves, and an ILO figure of 21 million held in forced labour.

Academics have questioned the new figure, saying it may be a compromise between the two groups, and could muddle efforts to tackle slavery and divide the anti-slavery movement, given the explicit inclusion of forced marriage for the first time.

Yet the ILO and Walk Free said they had made great strides in recent years in collecting more data and building upon previous methodologies, and that the new estimate would better inform anti-slavery actors of the true nature of the crime.

The groups conducted surveys in 48 countries and interviewed about 71,000 people - up from 25 nations and 42,000 respondents in Walk Free's 2016 figure - with findings supplemented by data from the UN's International Organization for Migration (IOM).

"This is a global estimate with important global findings, such as the fact more women than men are victims," said Fiona David, executive director of global research at Walk Free.

Almost three in every four slaves were women and girls and one in four was a child, with modern slavery most prevalent in Africa followed by Asia and Pacific, according to the estimate.

"The estimate wasn't intended to enable a certain country to set up a precise action plan to combat slavery," David added.

ETHICAL CONCERNS

The methodology pointed to several limitations of the data, such as the difficulty of recording cases of forced sexual exploitation, measuring all forms of human trafficking, and carrying out surveys in Central Asia and the Gulf region.

"We need more data on the Middle East," said Nick Grono of the Freedom Fund, an international initiative to fight slavery.

"As you know, you cannot simply walk into a labour camp in Qatar and start conducting interviews with migrant workers. It is a fast way to get deported," added Grono, Freedom Fund's CEO.

The ILO and Walk Free said they would like to see victims of all forms of human trafficking, from drug to organ trafficking, included in future estimates, as well as children enslaved and sent into war, as better data becomes more readily available.

Yet this will require more money pumped into the drive to end slavery, and a wider range of data collection methods, said Kevin Bales, professor of contemporary slavery at Britain's University of Nottingham and a member of Walk Free's data team.

And seeking out data on the lucrative, ever-evolving and largely hidden crime of modern slavery also raises many ethical questions, and is often fraught with danger, activists say.

"It is very, very tough to get information on the commercial sexual exploitation of children," said David of Walk Free.

"What do you do when you find a victim while doing research? You can't just leave them there and continue doing your survey."