Exactly how many people live on our planet is one of those difficult-to-answer questions. The UN estimates is 8.2 billion, but that’s largely based on census data, which is certainly not a perfect measure.

So when a recent study from Finland found that rural populations around the world had been underestimated by 50 to over 80%, the media got quite excited. This would be a big error - a 50% underestimate would mean the actual number of people in an area is double the number they thought there were.

One newspaper in Spain - El Mundo - did its own sums and said this meant there were potentially 2 billion more people in the world than we currently think there are.

But is it what the researchers in Finland actually meant?

“Absolutely not,” says Josias Lang-Ritter, a researcher from University in Finland and a co-author of the study.

Tim Harford speaks to Josias to figure out the right way of understanding the study.

Presenter: Tim Harford Producer: Caroline Bayley Series producer: Tom Colls Production co-ordinator: Brenda Brown Sound mix: Nigel Appleton Editor: Richard Vadon