The Julian Assange extradition case has put Sweden's relatively high incidence of rape under the spotlight. But can such statistics be reliably compared from one country to another?

Which two countries are the kidnapping capitals of the world?

Australia and Canada.

Official figures from the United Nations show that there were 17 kidnaps per 100,000 people in Australia in 2010 and 12.7 in Canada.

That compares with only 0.6 in Colombia and 1.1 in Mexico.

So why haven't we heard any of these horror stories? Are people being grabbed off the street in Sydney and Toronto, while the world turns a blind eye?

No, the high numbers of kidnapping cases in these two countries are explained by the fact that parental disputes over child custody are included in the figures.

If one parent takes a child for the weekend, and the other parent objects and calls the police, the incident will be recorded as a kidnapping, according to Enrico Bisogno, a statistician with the United Nations.

Comparing crime rates across countries is fraught with difficulties - this is well known among criminologists and statisticians, less so among journalists and commentators.

Sweden has the highest rape rate in Europe, author Naomi Wolf said on the BBC's Newsnight programme recently. She was commenting on the case of Julian Assange, the Wikileaks founder who is fighting extradition from the UK to Sweden over rape and sexual assault allegations that he denies.