Kevin Bales, co-founder of Free the Slaves and well-known human trafficking expert, first estimated there to be 27 million slaves worldwide. This was an approximation offered in his 1999 book Disposable People. Since then, the topic of human trafficking has garnered international attention—yet for the past 14 years, the estimated number of slaves worldwide rested at Bales’ original approximation. But now, with a team of researchers at the Walk Free Foundation, Bales has introduced a new number: 29.8 million. The new “Global Slavery Index 2013” seeks to measure international slavery and human trafficking, and to provide informational tools for institutions fighting the problem.

Two potential weaknesses of such a report lie in its definition (how broad or specific it is, how easy to measure) and its methodology: how does one measure the global population of slaves worldwide, when slavery is an illegal and clandestine activity? The report’s authors explain their methodology, which focused primarily on secondary collection (via both governmental and non-governmental reports) and representative random sample surveys. Nick Grono, CEO of the Walk Free Foundation, told The Guardian, “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’s definition of slavery and human trafficking has received some skepticism. Bridget Anderson, Deputy Director of the Centre on Migration, Policy, and Society at Oxford, told The Guardian this report gathers “unjust situations” around the world and labels them as “slavery.” “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. Here is an excerpt from the report’s definition section:

In 2013, modern slavery takes many forms, and is known by many names. Whether it is called human trafficking, forced labour, slavery or slavery-like practices (a category that includes debt bondage, forced or servile marriage, sale or exploitation of children including in armed conflict) victims of modern slavery have their freedom denied, and are used and controlled and exploited by another person for profit, sex, or the thrill of domination … The chains of modern slavery are not always physical – sometimes escalating debts, intimidation, deception, isolation, fear or even a ‘marriage’ that is forced on a young woman or girl without her consent can be used to hold a person against their will without the need for locks or chains.

One can see Anderson’s point. Not only does this definition include a plethora of hidden, illegal criminality—it also includes criminality across a broad variety of platforms: the trafficking of persons across borders, private commercial labor, sex slavery, child soldier kidnappings, and forced marriages. Also, from reading Bales’ books and a variety of other books on the subject, I have learned “coercive labor” situations often do involve pay. But they involve pay in ridiculously minuscule amounts, offset by mountains of employer-determined debt. Thus, the “bondage” described is of a tricky and hidden nature.

The report’s definition is not necessarily wrong. It is good to have some broad (albeit sketchy) statistics on the issue. But Grono himself admitted “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.”

While child and forced marriage are awful human rights abuses, should they be included in the Global Slavery Index? Perhaps so—but consider, we now have a conglomeration of commercial, domestic, and sexual exploitation in the same dataset. How does one begin to parse a number so large? The index’s inclusion of basic law information for the top 10 worst countries in the index could be helpful—if one fights trafficking in Mauritania, Haiti, Pakistan or India. But this is a limited contribution.

This is not meant to be harsh—the report’s authors are working for a noble cause. But one hopes they can improve the index with time. Perhaps a next step would be to specify data according to definitional groupings. What if one was to apportion the numbers for each nation according to commercial, domestic, and sexual slavery (perhaps another category for child soldiers, as well)? It would require more work, of course, but this division would allow for more practical data offerings. 29.8 million is a horrid and shocking number. But it is also, unfortunately, a rather useless one at this point.