In the first place, I must state that I have not worked on this field and therefore must apologize for stepping into the discussion. However, I assisted a colleague of mine who studied the socio-economic dimensions of prostitution long ago; and, personally worked with destitute children pushed into begging and menial labor, some of whom were children of prostitutes. Keeping the impressions and experiences that I gathered during those times, I would like to list the following:

1. A fair portion of the persons involved are there not entirely against their will. Getting them to state facts actually scares them due the apprehension that they may loose their only source of livelihood, however dishonorable it may be in others eyes.

2. There are quite a few "self-appointed guardians of the innocent" in the guise of NGOs who will manipulate destitute women/juveniles in order to color their holier than thou picture. Data/facts/interviews from such sources could be highly unreliable. We had actually come across a "voluntary organization" who identified destitute children and channeled them into flesh trade. My apologies for saying this bluntly, but this side of the world is certainly not a decent place, let me assure you.

3. Victims of trafficking/child abuse/flesh trade have unfathomable scars in their minds. Expert and a truly empathetic counseling is needed to gain their confidence. As for people who ply the trade, even police interrogations/expert investigations stretching over many months or even years are unable to reach the bottom of their machinations. A researcher who is on a shoe string budget and threatening timeline will find it extremely difficult. In one instance when police allowed my colleague to interview prostitutes for his study, one of them stumped him by asking him to marry her so that he could at least pull one person out of hell. She taunted him by saying that a 300 page report of his interviews otherwise would amount to nothing more than a sheet of paper on which his masters degree would be printed!!

So, other than the skills and knowledge that may be needed for a study of subjects like trafficking, the researcher may need tons of courage to face such situations.

I hope this helps. Have a great day!! 

Threat to Life! Given the desperation of the key players, they can go to any extend in self-defense including threatening the life of any investigator (researcher). I consider this my greatest challenge in conducting research on human trafficking.

Inadequate protection of the victims or planning for their future.  

In a chapter in the Oxford Handbook of Organized Crime (Kleemans & Smit, 2014), we review what is known about human smuggling and trafficking for sexual exploitation. These phenomena are frequently confused, despite the differences. Human smuggling primarily relates to illegal immigration and the violation of immigration laws, whereas human trafficking primarily relates to exploitation. Furthermore, discussions about smuggling and trafficking are often clouded by generalizations, such as the cruel smuggler and the innocent, uninformed trafficking victim, lured into prostitution (e.g. Zhang 2009).

Unfortunately, the heated debates about human smuggling and human trafficking stand in contrast with the amount of sound empirical research on these issues. In the coming decade, we should find out more about the impact of different immigration policies and anti-smuggling initiatives. Opportunity-reducing initiatives against human smuggling may also have negative side effects, for example on true political refugees, travel conditions, and prices. More research on the financial aspects of human smuggling and human trafficking might also reveal much about the profits of different actors and the social organization of these illegal activities.

The debate about tackling trafficking for the purpose of sexual exploitation tends to be particularly heated and based upon emotions and beliefs rather than upon facts. Weitzer (2010) describes how prohibitionist individuals and organizations, who want to abolish the sex industry as such, because they regard it as universally harmful, use unjust generalizations of certain research outcomes and biased data collection methods, and distort or leave out inconvenient findings to make their point. According to him, this ‘oppression paradigm’ contributes to a mythology of prostitution that increasingly influences public policy.Much has still to be learned about the effectiveness of counter-trafficking initiatives, in particular, how do different prostitution and enforcement policies affect the opportunities and restrictions of various parties: clients, owners, operators, procurers, and prostitutes? The number of monitoring and evaluation studies in this area is rising, but still too little is known about which interventions have been the most effective in preventing human trafficking, protecting victims, and prosecuting traffickers (U.S. GAO 2007; Van der Laan et al. 2010), and which ones are not effective or maybe even counterproductive, causing collateral damage (Dottridge 2007).

Furthermore, we need to understand why some women do not report to the authorities or why they decline assistance. Therefore, more research is needed on the complex relationships between victims and offenders (see also a recent publication by Verhoeven, Van Gestel, de Jong and Kleemans, online first: European Journal on Criminal Policy and Research). Because existing studies mainly focus on victims who have been assisted, victim profiles may be biased. Much can be learned from other sources, such as field interviews and law enforcement investigations. Another important area for research is the victimization of minors, boys and girls ending up in prostitution, and the roles played by their social environment. Lehti and Aromaa (2006) point out that there may be serious underreporting of human trafficking in minors. Finally, research should focus not only on victimization, but also on human agency, particularly voluntary prostitution and conditions of (in)dependency, economic motivations, women making a career in prostitution, and women who become involved in trafficking as (co-)offenders.

The contested nature of both illegal immigration and prostitution, and widely opposing views on these matters seem to block sound empirical research on human smuggling and human trafficking.

The illegality of the underlying activities makes it hard to research. It's also the main reason that it's underground, which makes the victimization possible.

In addition to what Prof. Kleemans just said (which I very much agree with), I would like to point out lack of a uniform approach to THB reporting on national/subnational levels (which is further compounded by an overemphasis on trafficking for sexual exploitation). As a result, THB-related crimes may be recorded and reported under different crime classifications. Overreliance on self-reporting is another problem, especially where the victim belongs to a group traditionally viewed by the law enforcement in a negative light.

Not my area BUT I know Hazel Barrett and others at Coventry University in the UK have researched this and would be worth contacting for collaboration and/or info...

In any cases of missing person especially from the places where the trafficking rate is higher in consideration to other places , FIR should be lodged immediately by Police without making any delay. As this is one of the major problem in cases of trafficking for the registration of FIR, and ultimately  becomes fatal for the victim.

.As so many of you have stated, it is difficult to gain accurate data on the topic. I have pull statistics from a wide range of sources dealing with different aspects of the topic, but in many cases, the numbers reported are fairly broad-based estimates. I have found about the best I can do is establish trends

Scarcity of reliable data on the subject especially in the developing economies

Good submissions