Over the past couple of years, you’ve probably noticed the xG (Expected Goals) metric gain a lot of mentions in the media. Pundits and other experts constantly point to the relatively new piece of data as a way to back up their arguments or confirm whether a team’s results reflect the performance. But what does the information actually mean and how can it be used to aid your future betting activities?
Let’s take a closer look at football’s latest stat in greater detail now.
What Is xG (Expected Goals)?
The Expected Goals (xG) metric is ultimately one that curates he data relating to a team/player’s performance over the course of a game or season before calculating the amount of goals that a team should have scored.
You may encounter several terms when looking at the realm of Expected Goals. Here are some that you need to understand.
Expected Goals (xG): This figure shows the number of goals that should be scored by a team based on the quality and quantity of shots. Can also relate to both teams combined.
Expected Goals per 90 (xG/90): This figure shows the amount of goals an individual player should have expected to score in 90 minutes.
Non-penalty expected goals (npxG): This figure shows the expected number of goals, but excludes penalties from the equation.
Expected goals for (xGf): This figure shows the number of goals a team should score based on the quality and quantity of shots.
Expected goals against (xGa): This figure shows the number of goals a team should concede. Again this is based on the quality and quantity of shots by the opposition.
Expected goals assisted (xA): This figure shows an individual player’s expected assist tally based on the quality and quantity of shots laid on by their creativity.
Expected points (xPts): This figure shows the number of points a team should have won over the course of a season. This takes in the opposition of each match into account.
The xG figure is presented as a decimal value, which may seem a little confusing at first. After all, you cannot score 1.35 goals in a match, for example.
However, the figure is calculated by using an advanced algorithm that ranks the likelihood of a shot resulting in a goal. A shot with a 50% shot of being scored will be given a 0.5 rating. Therefore, every shot will be ranked between 0 and 1. All of the shots over the course of a match are added together to give a complete tally for the game. This hypothetical example should clear things up:
Arsenal play against West Ham, and take 10 shots during the match.
The algorithm shows that the probability of each shot becoming a goal are as follows;
20% (0.2), 7% (0.07), 3% (0.03%), 85% (0.85), 6% (0.06),
52% (0.52), 11% (0.11), 14% (0.14), 1% (0,01), and 28% (0.28).
When added together, Arsenal’s xG for the match is 2.27.
How Is xG Calculated?
The xG algorithm was put together by Opta, who analysed over 300,000 shots to find the statistical likelihood of shots being scored. When a player takes a shot, a variety of variables are taken into account. This includes type of assist, distance to goal, angle, and whether it was taken with the foot or head.
Individual player’s abilities are not taken into account, so this remains a truly unbiased and objective metric. If a player statistically has a 60% chance of scoring a specific shot, it will be reflected in the data – whether it’s Sergio Aguero or Sergio Busquets.
What Does It Mean For Analysis?
Understanding the basic concept of the xG metric is one thing, but appreciating what it means is another altogether. The data can show how a team is performing with regards to the chances they’ve created and conceded. Given that football is a low-scoring sport, this information can often provide far greater accuracy as to whether a team should be getting results.
More importantly, this is far more accurate than the ‘total shots’ tally as players may take shots from distance that have next to no chance of going in. This can skew the data while the xG rating stays true to the team’s performance.
While the result of an individual match might not always correlate to the performance, most teams will generally pick up results to reflect the performance stats over the course of a season. For fans and pundits alike, this can be very useful when analysing how well a team is doing.
For punters, though, the data can transform everything.
Using The xG Metric In Betting
The xG stats can be found at various websites including Opta and Understat.com. As a punter, you should almost certainly use it to your advantage.
Sometimes in football, teams or players start the team slowly. For example, Harry Kane had a reputation for not scoring in August until breaking that duck towards the end of August 2018. However, the fact that he was taking shots resulting in a high xG/90 rating suggested that he’d always come good – 59 Premier Leauge goals over two seasons underlines that.
Likewise, clubs often struggle to score for a few weeks. If they are creating chances, though, you can anticipate an upturn in fortunes at some stage. Conversely, when a team that is struggling at the bottom of the table is struggling on the xG ratings could continue to be in for a tough time throughout the campaign.
Bookmakers usually set the odds and lines in correlation to recent results, which may not always tell the full story. With the xG rating metric at your disposal, you can work the odds in your favour on many markets:
When you locate the xG stats for a specific league or tournament, you can even split the data into home versus away. Heading into the first international break of 2018/19, the following stats were available.
Huddersfield Town xG/90 at home of 0.32 V Crystal Palace xG/90 away 1.05.
So, the statistically suggest that the visitors should sneak a 1-0 win. Of course, we know that football doesn’t always work like that. However, if the hosts were deemed favourites for this game, punters could deem it worth placing a bet on the visitors to win.
Of course, there are always other contributing factors to consider ranging from team news to extreme circumstances such as red cards in previous matches. Still, the xG rating can be used to support your initial thoughts to confirm whether a bet is statistically a good selection.
In addition to the season stats, the xG from the previous head-to-head meetings can be telling.
Total goal markets are another popular option for fans of the xG market. This is because punters can access the expected goals against tally as well as the expected goals for figure. In turn, this can provide guidance for bets on the BTTS, Total Match Goals, and Total Team Goal markets.
Teams at the top level tend to revert to the mean over the course of a season. Whether this means that their current goals tally will improve or decrease depends on how their performances correlate with recent results. Either way, though, you can be quietly confident when making selections based on the xG values.
By no means is the xG information a guarantee that your results will come in. After all, the discrepancy between these figures and actual shorelines shows that anything can happen in football. Nonetheless, a team that creates a greater number of chances, particularly high quality opportunities, should score more goals than one that doesn’t.
For that reason alone, you should always analyse this metric to support your football betting selections. If it’s good enough for the clubs and the media, it’s certainly good enough for you.