Stats Busters: Are away goals as/more important than wins in two-legged ties?

One quirk of soccer, and particularly European soccer, is the two-legged ties that are used in cup competitions. Playing a game at the home venue of each team provides a theoretical neutral playing field for both teams. That said, a common commentator adage in soccer is that in such ties, away goals are vital, and that a 2-2 draw can be better than a 1-0 win away from home.

The Logic

In European Soccer, and other competitions worldwide, in order to encourage more fluid play and reduce the number of ties decided by the lottery of penalties, they introduced the 'Away Goals rule'. This rule essentially means that if a game is tied after 2 legs, the teams are first separated by whichever one of them has scored the most goals away from home. With Spurs drawing 2-2 in Turin, if Juventus had drew 1-1 with Tottenham at Wembley, Tottenham would have gone through on the away goals rule. If Juventus drew 3-3, Juventus would have gone through on the away goals rule. If it finished 2-2, we'd have played extra time and penalties. So, away goals matter. But how much?

The Scientific-y Method

There have been roughly 1000 two-legged ties between the Champions League, Europa League and the EFL Trophy (currently the Carabao Cup) over the past 7 years. Collecting data from each of those, we isolated both the margin after the first leg (e.g. -2 means the away team trailed by 2 goals after the first leg), and the number of away goals scored.

Teams go through the majority of times if they draw or win the first leg - but winning is better.

If the away team won the first leg, then they won the tie over 90% of the time. A draw in the first leg was also a decent result, with the team going on to win the tie 66.6% of the time. Avoiding defeat in the first leg was crucial though, with a team losing away from home left with only a 40% (-1) or 18% (-2) chance of advancing.

If you scored in the first leg, you had a good chance of going through. Score 3, and you go through 90% of the time!

Obviously to win away from home you have to score, and that's reflected in this graph. A team who failed to score went on to win only 38% of the time (and the vast majority of those were 0-0 draws), while a team scoring 1 advanced 57% of the time, rising to 90% if you score 3 away from home.

Let's look closer at the tied outcomes though:

If you score 2 goals, the outcome is still crucial — winning is better than drawing.

Teams that draw 2-2 in the first leg away from home end up advancing 63% of the time - so that would have been good news for Spurs supporters - but crucially those who win the first leg scoring 2 goals advance at 90% or better.  Similarly, teams who score 1 goal in the first leg and draw advance 64% of the time, while those who win advance 87% of the time.

Scoring 1 goal is only marginally different than scoring 2.

Result

And, really, that's all we need to put pay to this one. Draw the first leg 2-2, even with your two away goals, and you move forward 63% of the time. Win the first leg 1-0, and you'll be playing in the next round 86% of the time. Winning 2-1 vs 1-0 only gets you a 4% improvement in your advancement rate.  Away goals are nice, but 2-2 isn't much better than 1-1 (though both are better than 0-0), and neither compares to getting the away win.

SI Verdict: Busted