By Manav Tanneeru, CNN

Slavery still exists. Of that there isn’t much dispute, if any. But how widespread is what many experts call modern-day slavery?

Estimates range from about 10 million to 30 million, according to policymakers, activists, journalists and scholars.

The International Labour Organization, an agency of the United Nations that focuses on, among other things, labor rights, put the number at a “minimum estimate” of 12.3 million in a 2005 report.

Kevin Bales, a sociologist who serves as a consultant to the United Nations and has authored several books about modern-day slavery, estimated the number was 27 million people in his book “Disposable People: New Slavery in the Global Economy.” The book was published in 1999.

There is yet another estimate. Siddharth Kara, a fellow on trafficking at Harvard University and also an author, recently told CNN that his calculations put the range between 24 million and 32 million. That number was current as of the end of 2006, he said.

There are several reasons behind the variance in numbers, said Ben Skinner, who published a book about modern-day slavery – “A Crime So Monstrous: Face-to-Face with Modern-day Slavery.”

“There are two big problems with the count,” Skinner, a Senior Fellow at the Schuster Institute for Investigative Journalism at Brandeis University, said during a telephone interview. “The first is that the people we are counting are, by definition, a hidden population.

“The second problem is more of a theoretical one where the definitions are not in place. We don’t have a common definition still as to what slavery is.”

‘A hidden population’

Slave labor has been a part of civilization for much of history. It was practiced openly and its legality wasn’t much of a question. During the height of the trans-Atlantic slave trade, its scale was carefully documented.

Today, slavery is illegal in every country. Yet it persists, in secret, exploiting the poor and the marginalized – which poses immense challenges for legal authorities, activists and experts working to track the problem.

Skinner recounted a conversation he had with John Miller – the former State Department ambassador at large on modern slavery from 2002 through 2006 during the George W. Bush administration – about the inherent difficulty of counting a population that is difficult to find.

“These are not people that stand in line, raise their hands and wait for the census to be taken,” Miller told Skinner.

And, even when found, they may not want to be identified, Skinner said. “They are victims of a crime and that is still oftentimes missed as a crime,” he said.

The enslaved may be involved in prostitution or might be in a country illegally as a result of trafficking – activities that could land them in trouble with the law. So, they’d rather keep quiet about their condition, Skinner said.

“They are individuals who will be seen as perpetrators of a crime against the state rather than victims of a crime against humanity,” he said. “They are aware of that so they don’t self-identify.”

It also isn’t the easiest thing for observers to get data from countries about how big of a problem slavery is within their borders.

For example, South Asian countries will acknowledge problems with sex trafficking because of a perception that it’s not just a South Asian issue, Skinner said, echoing a theory from John Miller.

However, they may not be as forthcoming about their problems with debt bondage – when someone has to pay off a loan through work and may be trapped in the situation because the amount earned is too little to pay off the amount of money borrowed.

“There’s a self-perception that debt bondage is a rather embarrassing part of the continuing underdevelopment in parts of their countries,” Skinner said.

Definitions and divisions

Before you can count something, you have to define it, and a broadly accepted definition of what modern slavery encompasses has been elusive.

In 1926, a treaty signed in Geneva under the auspices of the League of Nations, the precursor to the U.N., defined slavery as “the status or condition of a person over whom any or all of the powers attaching to the right of ownership are exercised.”

The ILO, in 1930, used the terms "forced or compulsory labor" to describe “all work or service which is exacted from any person under the menace of any penalty for which the said person has not offered himself voluntarily.”

Roger Plant, who worked at the ILO from 2002 through 2009, said during a telephone conversation that forced labor is “when you get into work or service without the freedom of choice and you can’t get out of it without punishment or the threat of punishment."

Kevin Bales offered this description: “To me slavery means one person who is completely under the control of another person, that they use violence to maintain that control, they exploit them, make money out of them, and that the person just can’t walk away.”

There is, then, the term “human trafficking,” which is sometimes used interchangeably with the word “slavery.” According to the U.S. State Department, “human trafficking” describes “activities involved when one person obtains or holds another person in compelled service.”

The State Department says the term includes sex trafficking, forced labor and bonded labor. It also includes, among other things, the use of child soldiers and forced child labor.

The terms and their meanings seem straightforward, but the divisions come to light when legislators try to reconcile the definitions with their country’s situation.

“Within the trafficking community, there really isn’t a consensus on what slavery means,” Skinner said. “That’s harmful, that’s detrimental.”

The biggest consequence of incorrect data, not knowing the full scope of the problem or where it’s concentrated can lead to poor decisions on where to focus resources and how best to solve the problem, Skinner said.

“Slavery, on its face, is monstrous,” he said. “I think it’s important to be motivated by emotion but to, very quickly, come to the point of getting to the cold, hard business of figuring how best to free as many slaves as possible.

“Part of that is understanding how many slaves there are and understanding where they are."