Choosing the best news photo of the past year is not a science.

The winner is chosen because it captures a major story of the year in a unique and original way that triggers a visceral response in the beholder, said World Press Photo project manager Erik de Kruijf.

There is really very little difference in technical quality between winning photo and the others chosen for the exhibition, which is on now at the Canadian War Museum, said de Kruijf.

“It is very hard to say number one is a little bit better than number two,” he said.

However, an international jury must select a winner from the thousands of submissions.

This year, they selected a photo by Italian photographer Pietro Masturzo of women shouting from rooftops in Tehran to protest the results of the Iranian presidential election.

The winning photo was selected because it tells a story of the protests in an unique and innovative way, said de Kruijf.