Will an away goal prove decisive in UCL Semi-Final?

Will an away goal prove decisive in UCL Semi-Final?

By Mark Taylor Apr 25, 2014

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With both 2014 Champions League Semi-Finals finely poised after the first-leg we use Poisson analysis to evaluate how decisive an away goal could be. Are Chelsea at a disadvantage despite a 0-0 draw in Madrid and how advantageous was Real Madrid’s 1-0 home win against Bayern?

Lack of goals follows historic pattern

The UEFA Champions League has almost reached its conclusion following the completion of the semi-final first legs. In keeping with the later stages of many knockout competitions, goals were rare. Since the turn of the century, twenty-one semi-final 1st leg ties have produced under 2.5 total goals and just nine have gone over. Therefore, a goalless stalemate in Chelsea’s visit to Atletico and a narrow single goal victory for Madrid over Bayern wasn’t a departure from long-term trends.

The lack of goals, especially in the Chelsea/ Atltico means an away goal may play a vital role in deciding the ultimate outcome of the tie and therefore, its ominous presence will influence the odds for each side to progress to the final match.

Understanding the away goal

Away goals where the aggregate score is tied apply both at the end of normal time in the 2nd leg and at the conclusion of extra time, should it be required. The fairness of the away goals rule has been questioned in the modern game, most notably by Arsene Wenger following his side’s elimination by Bayern Munich in 2012/13’s round of 16, when away goals broke an aggregate 3-3 score line in favour of the eventual winners.

Fair or not, both sides are aware of the rules and have the same opportunity to prioritize their attempts to claim an away score. Also, in the event of extra time being required in the 2nd leg, the opportunity for the visitors to score an away goal in that period counterbalances the extra thirty minutes of home field advantage received by the home side.

The initial aim of the rule was to encourage attacking play by the away side. However, the flip side of this is that home teams have increasingly become wary of conceding at home in 1st leg ties, so they in turn have become more cautious. This occasionally results in a sterile stalemate, such as Tuesday’s game in the Vicente Calderon, where two counter attacking teams were unwilling to take too many risks.

Poisson analysis of Chelsea vs. Atltico

The UEFA website sent mixed messages to Chelsea fans following their 0-0 draw in Spain. On the one hand, by virtue of avoiding defeat, Jose Mourinho’s side were credited with being handily placed, but later doubt was cast on their prospects because of their failure to profit previously from a 0-0 1st leg score line in Barcelona at the same stage in 2008/09.

In an attempt to explore Chelsea’s chances we can use a Poisson approach to break down the likely match outcomes next Wednesday and track which ones favour Chelsea and which suggest Atltico will progress.

Atltico currently top La Liga, while Chelsea are likely to finish third in the Premiership. Therefore the Spanish team could be legitimately considered as slightly superior of the two teams and a comparison of the respective odds for a home win in each of the two legs appears to confirm this.

Atltico were given around a 49% chance of beating Chelsea on Tuesday, but Chelsea are only around 43% to win the return at Stamford Bridge on Wednesday. However, the fact Petr Cech will miss the 2nd leg and John Terry is also a major doubt, must be considered

A Chelsea victory over the 90 minutes on Wednesday will see them through to the final, but progressing following a 90 minute draw is much less certain. Chelsea’s lack of an away goal ensures Atltico progress in the case of any score draw, while a 0-0 will send the game into extra time.

Therefore, we need to break the overall draw probabilities down into their correct score components and the Poisson distribution is a fine framework to achieve this. The Poisson has been described here, but basically we take the average number of goals we expect Atltico or Chelsea to score on Wednesday night and project probabilities for each side to score a particular number of goals in the match.

Subject to a few slight tweaks, such as the Poisson slightly underestimating the likelihood of drawn games, we can combine the chances of neither team scoring to arrive at the chances of the match ending 0-0 after 90 minutes. We can do likewise for the score draws and we can therefore use these probabilities to see the impact of the goalless 1st leg on Chelsea’s prospects.

Overall, the chance of the match ending level after 90 minutes is around 28%. For two closely matched sides around 30% of these drawn matches will be goalless and this merely keeps Chelsea in the tie via extra time. Nearly 70% of the draws will see Atltico progress, with 1-1 being the most likely outcome.

The disadvantage caused by their failure to score in Spain continues if the tie moves into extra time. Chelsea’s chances of winning after extra time fall to below 30% and a continued stalemate has a 50/50 chance. They do however have the comfort that this time 80% of the draws will be goalless, allowing the game to go to an evenly contested penalty shootout.

We now have a sequence of score line based match events, each with a Poisson derived probability attached, which lead to either Chelsea or Atltico qualifying for the final. Despite avoiding defeat in Spain, Chelsea’s lack of an away goal has handed their opponents an advantage on Wednesday, should the game remain tightly fought.

Chelsea’s possible routes to the final are to win the 2nd leg in 90 minutes (about a 43% chance). Draw 0-0 after 90 minutes and win in extra time (about a 2.5% chance) and a 0-0 stalemate throughout regular and extra time and a win on penalties (around a 1.6% chance). That is enough, once totalled up to give them a 47% overall chance to book their place in Lisbon, with Atltico slightly ahead at 53%.

UEFA.com was correct to temper initial optimism with a later air of caution, even though they based it on a single match played six years ago.

Real hold the advantage after clean sheet in Madrid

The process can be repeated for the other semi-final involving Real Madrid and Bayern Munich. Madrid’s 1-0 victory gives them an edge that Atltico do not possess and in addition, by preventing Bayern from scoring an away goal, they can claim all but one of the one-goal margin defeats in the 2nd leg for themselves. The exception being a Bayern repeat of the 1-0 home win which takes Madrid on a journey that may also end in spot kicks or possible victory by means of an extra time away goal.

Madrid will progress with all of their 90 minute wins in the 2nd leg, all of the draws and a sizeable chunk of the one goal defeats. Three more successful routes via extra time exist, each adding a couple of percent to their more substantial chances to win the tie in normal time. Despite Bayern being odds on to win the 2nd leg, Madrid are approaching 60% to win the tie outright and progress, especially with their counter attacking ability.

Click here to see the latest Champions League Semi-Final 2nd leg odds.

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