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The numbers behind Barcelona's momentous comeback win over PSG

Down 4-0 on aggregate, Barcelona became the first team in Champions League history to overcome a four-plus-goal deficit in a knockout-round match by defeating Paris Saint-Germain 6-1 in their round-of-16 second leg on Wednesday.

The only team to overcome a three-goal deficit was Deportivo La Coruna in the 2003-04 Quarterfinal against AC Milan.

How big is this win?

From a gambling and probability perspective, according to Westgate Las Vegas Superbook, Barcelona were 40-1 entering Wednesday to win the Champions League. They were 3-1 favourites before the round of 16 began. In addition, Barcelona, being down 4-0 from the first leg, were 10-1 to advance on Wednesday.

According to FiveThirtyEight.com's Soccer Power Index, Barcelona had a 7 percent chance to advance and a 3 percent chance to win the Champions League.

How did it happen?

Barcelona needed four goals just to level the aggregate score and send the second leg to extra time. In the first 50 minutes, Barcelona got goals from Lionel Messi, who scored his competition-leading 11th of the season; Luis Suarez; and an own goal from Layvin Kurzawa.

Suarez's goal was a good omen. Barcelona entered the day with 67 wins and five draws when he scores in all competitions.

But then ...

Edinson Cavani scored his 35th goal in European and league competition this season, the first player to do so in Europe's top five leagues. With the away-goals rule, Barcelona now needed six to advance.

In Champions League knockout history, only seven teams had scored six or more goals in the second leg.

So Barcelona made history

Barcelona then scored three goals from the 88th minute onward, including two from Neymar. They are the first team since 2010 to score three goals in the 80th minute or later in regulation of a Champions League match.

Barcelona have won all 21 games in which Neymar, Messi and Suarez score in all competitions, including three of the past seven games.

And this means

Barcelona are now the favourites to win the Champions League, at 32 percent after Wednesday's win, according to FiveThirtyEight.com's SPI. Superbook has Barcelona as 5-2 favourites to win the entire tournament.