Each year for the past three years, the anti-trafficking organisation Walk Free has released a Global Slavery Index - that is, an estimate of women, men and children around the world living as victims of modern slavery and human trafficking.
Their latest figure was announced last week: 45.8 million people. That's the entire population of Canada and Sweden - combined. It's nearly double the population of Australia.
It is a staggering number of people living without their freedom.
Oddly, though, last year's figure was "only" 35.8 million. Before that, 29.8 million. Why these vast leaps of five and ten million people?
Let's say you were trying to estimate the extent of every cave system on Earth - and those systems were constantly changing. No matter how much time and energy you threw at the problem, whatever number you produced would be wrong.
As your tools and techniques improved, you'd get closer to the truth - but you would never know the whole story. It's not only possible, but highly likely, that this year's figure - 45.8 million victims - still falls far short of reality.
In 2014 and 2015, I spent almost seven months investigating human trafficking in northern Vietnam. I heard countless stories from survivors, and victims' families. Locals often gave me their estimates of the numbers of girls and women disappearing from their villages.
I'd say I knew as much as anyone about the scope and mechanisms of human trafficking in that particular area, at that point in time. And yet, if you'd asked me for an estimate of the numbers involved, I couldn't have even guessed.
All I could have told you is that the problem is colossal - much, much bigger than you might imagine. And I was only looking at one piece of it - young women being trafficked into China for sale as wives and prostitutes - which is certainly not the only form of human trafficking in that area.
Much of what I learned about human trafficking in northern Vietnam came from Michael Brosowski, the founder of Blue Dragon Children's Foundation, a remarkable organisation with whom I've had the pleasure to work over the past three years.
In January 2014, when I interviewed Michael for our feature-length documentary, Sisters For Sale, one of my first questions was, 'Are there any reliable statistics on human trafficking in Vietnam?'
The short answer was, 'No.'
Michael then elaborated: 'It can be very difficult to draw a distinction between trafficking and general movement of people, so there's a lot of trafficking in the country that never gets recognised as such. People will say, this girl or boy went with a friend or aunty to live in a different part of the country, whereas they may well have been taken to work in a brothel or garment factory in slavelike conditions'.
When you take Michael's comments from one small country and apply them to the world at large, you begin to see how complex the problem really is.
With increasing globalisation, and increasing population, people are now moving around the world in greater numbers than at any point in history. Looking at air traffic alone, there are now 50,000 flights with over 8 million passengers every day.
That's 3 billion passengers every year - and how many more travel by train, bus or car? Who's to say how many of those people are being deceived, or coerced, and have their freedom taken from them? And, despite the name, you don't actually need to travel anywhere to become a victim of human trafficking.
I don't envy Walk Free their task of trying to measure the immeasureable. I don't envy the anti-trafficking ground forces their task of stopping the unstoppable. I don't believe human trafficking is an issue we can ever eliminate entirely.
But we can make a difference - every single one of us.
All we can say for certain is that human trafficking is an issue of inconceivably huge proportions. It's one of the world's fastest-growing criminal industries, and we need to do all we can about it.
Because whether it's 35 million, 45 million or even 100 million people, those aren't just numbers. They're real people with names and faces, and families they may never see again.
As Michael pointed out, 'I would argue it's not a good use of our time trying to gather that data. We know young people are being trafficked. My focus is on how to stop that from happening, and how to get them back.'
The first step is education - recognising the problem, understanding how it works, and telling others. That's why, as a filmmaker, I've given the past three years of my life to raising awareness of the issue.
Sisters For Sale will be a powerful and very personal exploration of the complex realities of human trafficking. It's the story of the search for, and attempted rescues of, my trafficked friends from Vietnam.
The film features interviews with victims and survivors, their families and friends, and experts in the field, including Michael. It showcases some incredible footage - including a real-life abduction, powerful reunions, and the wedding DVD of a forced marriage in China.
I'm pleased to announce today the release of a new extended trailer for the documentary. As with the original trailer, it features the talents of singer Rami Shaafi, musician Mike Taylor, and voice artist Betheny Zolt.
You can see the new trailer below, and support my work by pre-ordering Sisters For Sale here.
(Can't see the trailer? Click here.)
I'd like to thank those of you who have worked with me over the past weeks, particularly Belinda Bauer, Matthew Glen, Sammantha Mavin, Anthony Molyneaux and Kerstin Raitl.
If you haven't already, check out our new website here.