The most anti-statistical statistic, courtesy of Kristof

It's a good thing for Alex Tabarrok to draw attention to Nicholas Kristof's article on modern-day slavery in Cambodia (link), but what he calls the "arresting statistic" should really be filed under "Sentences to ponder".

Here is the statistic:

By my calculations, at least 10 times as many girls are now trafficked  into brothels annually as African slaves were transported to the New  World in the peak years of the trans-Atlantic slave trade.

I call this an anti-statistical statistic.

Why?

• There is no statistic! In spite of his "calculations", he never told us the final count. In the last paragraph, he implied that the unit is "millions". Given that he sourced himself for this statistic, one would hope to at least see what the number is.
• The only number in the "statistic" is at least 10 times. Unless he assumes that his readers know by heart the number of African slaves, we have no way of knowing if that is a big number or a small number.
• There is no indication that he controlled for population growth. Recently, it has been remarked how quickly world population has grown. There are more than 4 times as many people now as in the 19th century.
• Comparing "annually" to "peak years of" weakens an otherwise potent statement. I'm sure his point still stands if the African slave trade volume is expressed as an annual average rather than a multi-year aggregate. Why introduce an element that looks suspicious?
• I'm not sure the comparison to the African slave trade is relevant. By juxtaposing them, he equates the suffering of one to that of the other.
• The other problem with the Asian slavegirl- African slave comparison is the base population: from this population pyramid, one might guess that roughly 20% of Cambodia's population fall within the age and gender definitions for this slave trafficking, and that would be roughly 2.8 million. The base population for the slave trade involving black people in Africa is a vastly different calculation. The more appropriate statistic is what proportion of these women are part of the slavery.