Decoding La Liga's goal-scoring secrets
Beyond the goals we celebrate, there is a hidden narrative. A narrative that is woven with data, probability, but how can we measure a team's true goal scoring efficiency on the pitch? This is where a powerful metric comes into play: expected goals, or xG.
To understand this blog's explanation, we must first explain what expected goals, represented by the abbreviation xG, is. In essence, xG assigns a probability value to each goal attempt in a match. Each shot, header or action in the box is evaluated according to its probability of becoming a goal.
Is the one who scores the most goals the most effective? The answer is not always found in the simple goal tally. Some teams may be exceeding their expectations, while others may be below what their goal statistics suggest. Subtracting goals scored minus expected goals (xG) gives us a deeper and more accurate perspective on their ability to convert chances into goals.
Using the data obtained from FBREF.com we have produced the following graphs. Firstly we plot the ratio of "Goals- xG" against 90 minutes of play.
The case of Almeria is particularly striking: they are the fifth team that generates the most xG. They generate 1.60 goals on average during the 90 minutes and score 1.30 goals. Despite this, they find themselves in last place without a win after 10 games. It is a clear example that defences also win games.
On the other hand, we see that teams at the top of the table like Atco.Madrid, Girona and Real Sociedad are scoring more than expected.
If instead of analysing the ratio of goals around the 90 minutes, we make a cumulative analysis of the 10 matchdays on the difference between goals scored and xG, again the three teams mentioned above appear leading this analysis.
Atlético de Madrid: has scored 6.6 goals more than expected, this translates into 0.730 more than generated during the match.
Girona: 6.30 goals more than expected, which translates to 0.630 more than generated during the match.
On the other hand, the negative side of this analysis is on Alavés and Celta de Vigo.
Las Palmas find themselves with a cumulative 3.90 goals below expectations. Despite this, they sit 12th in the standings, 5 points clear of the relegation places. Despite being a very complete team both offensively and defensively, we must highlight the performances of Álvaro Vallés, one of the best goalkeepers so far this season.
In conclusion, we show you four frames of plays that end in goal with a low probability, demonstrating the effectiveness in front of goal that we have seen previously through the data.
* Own graphs (source: FBREF) and images taken from Longomatch. *
Author: Rafael Moreno del Río.