Going In-Play (Part 2): Goals vs. xGoals per Segment

Thanks for joining me on the second part of my in-play goals journey!

I hope you found last week’s blog useful for your weekend trading, as I was certainly happy with how the majority of the games panned out versus what I expected to see.

I’m going one step further this time around and will analyse each team’s average goals per 15 minute segment relative to their expected goals. My hope here is that you’ll be able to get a broader understanding of not just when teams score and concede goals and chances, but how frequently they take the opportunities they create, and when they are converting them better or worse than expected.

For this week’s blog, I’ve picked three games that will be broadcast live on British television this weekend. Some of the teams featured may be familiar from last week, but seeing as it’s a new gameweek, we’ll be looking at the inverse of the patterns we looked at last time out. So for instance, last week we were treated to a goals-fest when Liverpool played Bournemouth away, but this week, Liverpool will be playing at home, so I’ll be making sure to give you a novel view on Goals so far this season.

As mentioned, I’m going to be investigating an enduring hot topic in the analysis community in “expected goals” (defined by us and many as “xGoals”). xGoals is, as the name suggests, the number of goals a team is expected to score based on the number and quality of chances they create. I’ve previously discussed how we at StrataBet calculate this, but for a quick recap it’s important to remember that the granularity of StrataData allows us to combine chances not scored with those that are scored to calculate what a team is expected to score given the overall number of opportunities, weighted by the quality of each chance.

My colleague Dave Willoughby has previously written a great blog post on how we rate our chances, which I’d again encourage you to read, but briefly, I’ve calculated xGoals using the average conversion rates of our six chance categories, labelled Superb (~75% conversion), Great (~40%), Very Good (~25%), Good (~15%), Fairly Good (~8%) and Poor (~2%), summing a team’s total chances per game by conversion rate rather than by category. This has allowed me to weight each team’s chances and goals by the quality of each opportunity, in order to provide a firm xGoals number.

I’ll be using xGoals to identify when teams are over-performing and under-performing, by creating tables where you’ll see have some numbers coloured either red or green. These numbers are the difference between xGoals and actual Goals (xGoals minus Goals). The difference can be positive or negative of course, but the interpretation is different depending on whether the team is scoring or conceding. For instance, if a team is scoring (“goals for”), then a negative number is good, as it means the team is scoring more than they are expected to (i.e. a team is finishing the chances they are presented within that time period). However, when looking at conceding goals (“goals against”), negative numbers are bad, because this means a team is conceding more than they are expected. I’ve highlighted these metrics in each of the graphics below to help you identify these moments.

So let’s jump straight into the first fixture and put this into practice…

LEI MAN.png

Leicester City vs. Manchester City (Saturday 5:30pm, BT Sport 1)

Champions Leicester City welcome Manchester City to the King Power on Saturday and both teams could do with a good result considering their recent stutters. With Man City sitting on a -0.5 handicap, there doesn’t seem to be much in the market to separate the two after the suspensions for Aguero and Fernandinho, while Schmeichel and Drinkwater remain sidelines for the host. To begin, we can take a look at the game play data to help us predict what the outcome will be here.

Focusing the on the top left graphic, this season Leicester appear to be slow starters at home, having yet to score inside the first 30 minutes of a home match (Jamie Vardy has come closest to this mark in gameweek 2, when he scored in the 32nd minute). Ironically, Man City are leakiest away from home in the first 15 minutes, conceding just over 0.4 goals in the first 15 minutes of away games this season. If Leicester are to capitalise on this, they’ll have to capture some of their form from last season where they averaged 0.2 and 0.3 goals in the first and second 15 minute segments respectively!

Despite the expected slow start, Leicester can be positive after the first 30 minutes. This period is marked by a strong scoring period for the hosts and a bad conceding period for the visitors, so much so for Man City that they concede 0.15 goals more than expected (table on top far right). If Leicester can’t convert some of their chances (top right figure) in this period, they can still hope that their better than expected performance in first half added time can reward them with a goal, where they are over-performing (i.e. converting more goal scoring opportunities) by 0.08 goals (table on top far right, segment 45+).

On the flip side and completely unsurprisingly Manchester City appear to be more dangerous than the league average when away from home (bottom left graphic), as well as over-performing more than expected in 4/8 time segments (see green numbers in table in the bottom right graphic). Of these segments, the spell between 76 and 90 minutes could define this game. Whilst Man City are scoring more than expected, Leicester are conceding 0.1 more than expected, making this time segment the leakiest in their home games this season. So if the scores are level after 75 minutes, it might be a good idea to look at the price of Pep Guardiola’s side!

ManTot.png

Manchester United vs. Tottenham Hotspur (Sunday 2:15pm, Sky Sports 1)

Sunday afternoon sees Tottenham travel north to take on Jose Mourinho’s Manchester United side. With a handicap of -0.25, there isn’t much at all to separate these two other than home advantage, so hopefully the data will help us uncover some useful trends. Indeed, I’ve picked out three key moments in this match that you should certainly be looking out for.

For starters the opening 15 minutes look interesting, but mainly from Tottenham’s point of view. Focusing on the bottom left graphic, it appears that Spurs start strong away from home, a fact that can be coupled with Man Utd conceding the same number of goals at home as Tottenham score away (0.3). This is further evidenced by both team’s expected performance, because while Tottenham are scoring 0.17 goals more than expected, United are conceding 0.04 more (table on the bottom far right, 1-15). So don’t discount an early away goal here!

Manchester United generally peak between 31 and 45 minutes, scoring most of their home goals this season in that period, with an average just shy of 0.8 goals. This is Manchester United’s best spell when playing at home and they are over-performing by 0.22 goals (table on the top far right, 31-45). Mourinho can be proud of this, as not only does this stat way surpass the league average, but it also surpasses his predecessor (Louis Van Gaal), who only averaged 0.16 home goals last season in this same time period! This coupled with Tottenham’s propensity to concede most of their away goals in this segment (under performing the league average by 0.17 goals), bodes well for United.

Importantly we can expect some more twist and turns going into the second half, regardless of game state. This time however, Tottenham maybe able to impact heavily upon the Total Goals line, as at the beginning of the second half (bottom left graphic, 46-60), Tottenham peak with an average just shy of 0.6 goals. This appears to be a key period for Mauricio Pochettino’s men when they are away from home, as they’re over-performing by 0.25 goals (bottom right graphic and bottom right table, 46-60). If Tottenham are able to convert any of the chances they create in this period, then we may see the total goals line of 2.25 surpassed before the 60 minute mark!

LIVWHU.png

Liverpool vs. West Ham United (Sunday 4:30pm, Sky Sports 1)

This fixture looks set to be very volatile, particularly when looking at Liverpool’s attacking explosiveness at home (top row graphics), and with the total goals line set at a meaty 3.25, it would still not be a huge surprise to see this one surpass that. I’ve picked out a few key moments from this contest that I think may be instrumental in determining its outcome.

Focusing firstly on the top left graphic, Liverpool are naturally strong at home, generally surpassing the league average goals mark over the course of the 90 minutes. In the first 15, Liverpool peak in terms of their average number of goals scored, scoring 0.83 between 16-30 minutes at home so far this season. This is further evidenced by the fact that they are scoring 0.09 goals more than expected. Interestingly, however, it is in the period after this (31-45) where Liverpool are most efficient. During this segment, they over-perform by 0.39 goals, and while West Ham are consistently conceding 0.3 goals in the first 45 minutes away from home, this 15 minute period may also provide some first half goals.

I’ve also picked out two key moments in the second half that could sway the goals market. At the start of the second half (46-60 mins), Liverpool’s scoring and West Ham’s conceding rate are almost identical, with both hovering around 0.6 goals in that period. West Ham have done well so far this season to not concede more, particularly given the peaks in their xGoals graphic (top right graphic), but the data highlights that this is a period that Liverpool could exploit.

I’m also keen to see how the end of this fixture plays out. Looking at the data, we may be in for a late Liverpool goal. Between 76 and 90 minutes, both teams are expected to score (Liverpool) and concede (West Ham) more (top right graphic, 76-90 minutes). However, having said that, a Total Goals bet could even still be alive come injury time, where Liverpool are over-performing and West Ham are under-performing. This is evidenced by the table on the top far right, where at 90+ mins, Liverpool are scoring 0.26 goals more than expected, whilst West Ham are conceding 0.12 goals more than expected. With Liverpool handicapped at -1.75 and expected to win, the final 15+ minutes of the game may be instrumental in whether they beat the handicap or not.

West Ham on the other hand can take solace in their away performances between 61 and 75 minutes. They generally peak during this period, scoring just over 0.4 goals so far this season, but importantly they are over performing between 61 and 90 minutes for scoring when away from home. If West Ham are able to continue their form during this spell, we may be in for an exciting last 15 minutes to go with an exciting 75 before that!

Conclusions

Once again my hope for this post is that not only will it provide some interesting data driven observations for those of you who enjoy making in-play goal trades, but also highlight where teams are over-performing and under-performing relative to the chances they make and concede. This may be the difference between a team being exploited or exploiting another.

Last week we discussed how Jurgen Klopp’s Liverpool would need to be wary of Bournemouth’s attacking talent throughout the game, as Liverpool had already been conceding many more goals than he would like away from home. However this week, Liverpool may be in for an easier ride. Our analysis shows Liverpool are consistently outscoring the league at home, and importantly, creating opportunities to score – opportunities that marry up with West Ham giving teams plenty of chances to score against them.

Manchester City are naturally strong away from home and we may well see another early away goal from them in this fixture, though Aguero being absent will dampen this somewhat. Although Leicester are generally strong defensively in the first half, the 76th may be the turning point, as Man City’s scoring power peaks at this stage, while Leicester’s defence crumbles at this stage more than any other. In the other fixture focused on Manchester, the Reds will need to be prepared for Tottenham’s attacking threat, particularly at the beginning of each half.

With the inclusion of xGoals, I hope here you were able to see even greater detail about when a team is more dangerous or vulnerable at a particular stage of a game. I especially hope that this information will help you with your trading again this weekend. As mentioned last time out, stay tuned for some more in play analyses, investigating the relationship between Goals and xGoals depending on the game state, as well a journey through corner production and concession.

Until next time!

Sagar Jilka (@DrSagarJilka)

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