Expected goals have come into American sports a long time ago, but for several years now they have become an active tool in the football club analysis, so they can more easily identify their rivals. This tool has become so popular that its distribution has been steadily increasing. There are now freely available websites in which you can see accurate statistics on the expected goals in the top five leagues in Europe.
What are the Expected goals (xG)
In simple terms, xG is the probability of whether a given shot will result in a goal. What exactly is the quality of the shot and is the percentage of how likely it is to become a goal? This is done by special analytical companies such as Opta, Stratabet and others. They measure the quality of each shot on the basis of different components. In order to clarify this measurement, over 300,000 shots have been analyzed.
Each shot is made from a different distance of the post and from a different angle. For example, the smaller the angle, or the larger the distance, the lower the chance of a goal and this is measured in percentages. A number of other factors are also taken into account. Some of them are the type of shot (whether it is foot, header, from set-piece etc.), what is the position of the defenders. When we see a result in a match and then look at the statistics for the xG from both teams, we can judge whether and which of the two teams performed below, above or as expected.
The measurement for the expected goals has an index of 0 to 1. When a goalscoring chance is created, the index is formed, depending on all factors considered. If a goal situation has been created that has an index of 0.40, it means that the probability of a certain player scoring from that situation is 40%. In other words, every 10 shots from this position will result in 4 goals.
It is important to clarify that it does not take into account who is the player who strikes. If a full-back gets in a situation where the chance of scoring is high at the paper, we instinctively know that his real chance is far less than that of a central forward, for example. On the other hand, the most elite strikers score more goals than expected in certain situations. That’s what makes them so good.
Expected goals are divided into expected goals for (xGf) and expected goals against (xGa). There are variations of expected goals for without penalties and own goals (NPxG) or expected goals against without penalties and own goals (NPxGA), which gives an even clearer idea of what exactly happened in a given match under equal other conditions. For example, if a team has an index of 8.14 xG for 7 games and it has scored 18, but 7 of them are from penalties, the difference will be quite big.
After the expected goals there are also passes allowed per defensive action in the opposition half (PPDA), opponent passes allowed per defensive action in the opposition half (OPPDA), passes completed within an estimated 20 yards of goal (crosses excluded) (DC) as well as opponent passes completed within an estimated 20 yards of goal (crosses excluded) (ODC). There are other different variations for the types of passes or other parts of the game and there will be more in future.
There are also expected points in the long run (xPTS). The expected points for a team over a period of time are based purely and simply on the chances created and conceded and their quality in the matches, which basically means on the base of the xGf and xGa there is a certain algorithm which calculates the expected points (from 0 to 3) per game, after which it collects them.
How to use expected goals in betting
Do you know that one of the main tools by which bookmakers determine their odds is precisely the expected goals and points? It is very important to realize that the expected goals cannot predict the results of the matches themselves. There is no such tool in the world. If there was, most likely bookmakers will go bankrupt. Expected goals are simply a probability based on precision, accurate and voluminous statistics.
What this statistic can help you with is to guide you how the match was actually played, not the result itself. Thus, with the aggregate of more games, if one team has won only 9 points in 6 matches, and the expected points, as well as the expected goals scored in the matches, are more than they really were, then it is very likely that this team will improve its performance with over time to get closer to the real parameters. The more the data, the more real the estimate.
My advice is to use the expected goals with no other conditions and then take into account the factors that these statistics do not take. Factors that are not and cannot be taken into account are the psychological state of the teams or certain players, the events around them off the pitch, the new manager, injuries or transfers, and many more. However, this is why football is so interesting and unpredictable.
Most simply, the comments and discussions about which player and team how many goals they should have scored in a match can now be backed up with accurate and measured facts. Over time, statistics will improve more and more. However, this tool is primarily designed to be used with a specific context combined with tactical and video analyzes from the staffs of the teams themselves, in order to explore the rival or their own team.
I’ll finish with a quote from the great sir Alex Ferguson: “Statistics are like mini-skirts, shows a lot of things but does not show what is essential”. Well, in this line of thoughts now, through the expected goals, definitely the mini-skirts have been shortened a lot more and, accordingly, we see much more.
In addition, I would like to write about one of the factors these statistics cannot take into account and I mentioned above in the article. It’s the injuries and suspensions of the teams. That’s why me and the analysts who work in Betinum use mainly injuriesandsuspensions(dot)com for football team news and absences information, because they have it all. That’s one thing you just can’t ignore.
Guest post by: Svetoslav Kovachky, writer at Betinum
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