Wednesday, 31 August 2011

EP is just part of the Narrative.

Before we get carried away with the significance and application of EP in soccer,it's worth pointing out the limitations and pitfalls that are out there.A goal is usually a fairly significant event in a relatively low scoring game such as soccer,but it is just one event amongst many that occur during a game.Even the simple ticking of the game clock alters the EP from one minute to the next.So it's worth noting all the event that influence the course of a soccer game and try to quantify their relative importance.

Relative team ability.
The obvious first factor is the strength of each team.Often one is going to be a lot better than the other,even more so if the better team is also the home side.ManU are going to be favoured to beat WBA at the Hawthorns and they're going to be even bigger favourites in the reverse fixture.

Time Elapsed.
It's a fairly trivial problem to work out how many goals on average a team is likely to score against today's opponents at today's venue.It's also fairly easy to work out how that expected goal average decays as the game progresses.Armed with this information the relative win and draw percentages,and hence the EP can be tracked for both teams.

Red Cards.
Relatively infrequent (unless you are Arsenal),red cards can have a massive effect on a team's EP.Timing and current score is obviously a factor,but for example a team being reduced to 1 men just before half time can expect to score around half a goal less and concede half a goal more compared to the card not being shown.

Goals Conceded.
Another obvious contributor.A goal for the opposition not only raises their team's EP,but it also reduces that of their opponents.

So hopefully we can see that EP changes resulting from goal are just part of the story of the game.In relatively uncomplicated games such as smallish margin wins to nil against similar "on the day" strength of opponent,it can give a very accurate picture of how that goal has contributed to the seasons points total for that side.
However,if we use the raw EP from games where the lead is constantly changing then EP numbers become meaningless unless kept in context.

Let's look at last week's game of the week,Hamburger SV v FC Cologne.

Hamburger were relatively strong,home favourites.At kick off they would expect to pick up 1.79 points from this fixture if it could be replayed over and over again.After 10 minutes the game was still 0-0,Hamburger's pre game goal expectancy had reduced slightly,the draw had become slightly more likely,the home win slightly less and their current EP as now about 1.76.Time was elapsing and with the score still tied the home side was accumulating small amounts of negative EP.
Their 11th minute goal catapulted their EP from 1.76 to 2.45.Now a goal to the good Hamburger merely had to "draw" the remaining 80+ minutes of football to record a match win ("winning" the next 80minutes would course also get them a match win).This goal cushion saw their EP gradually rise as the game time elapsed.By the 20th minute it had risen to 2.46.They were now accumulating small amounts of positive "in game" EP.
Cologne then immediately equalised and Hamburger's EP went from 2.46 to 1.73.

And so it continued.Hamburger's EP drifted gently upwards or downwards in the periods of goalless play,while falling or rising precipitously when goals were scored by the sides.

Below is a table showing the amount of EP accumulated by both sides through the different phases of the game.

Game EP.
accumulated through Time Elapsed.
accumulated through Goals Scored.
lost through Goals Conceded.
Game EP.
Hamburger SV
FC Cologne

From these figures it becomes apparent why EP for individual goals can bear little resemblance to the points a player helps to gain for his team over the course of the whole season.EP is very strongly context driven,it can be very informative for individual players when applied to a single game,it shows how big a chance he gave his team to win that match.However,it's also more of a longterm measure of the type of side he's playing for.

Forwards playing for teams with poor defences can expect to accumulate EP seemingly in excess of those who don't.Continually playing from behind gives you greater opportunity to score high EP value goals.Hamburger SV's three goals were worth a combined EP of nearly three,but their points haul at the end of the day was zero.

Similarly a team by virtue of the lead constantly changing hands can score goals which have a combined EP that is in excess of the 3 points available for winning as in the case of Cologne verses Hamburger.

By contrast,Rooney has to score the first goal in a comfortable ManU win against inferior opponents if he wants to score a reasonably large EP.Once the first goal has gone in any subsequent scores will require a goal or two input from the opposition to make them worth much more than an EP of 2 or 3 tenths.

To make a player's goal EP a truely useful statistics this anomalies need to be addressed.Simply averaging a player's EP per goal for the season can be unfair to excellent players who have limited opportunities to score high value EP goals because their team spends large amounts of the game in very commanding positions.It is also plainly illogical that a player can gain more total EP in a game than there are points on offer,as would have been the case if one player had scored all of Cologne's goals against Hamburger.

High EP goals are certainly special goals,but whether that confers anything extra special onto the players who score them is open to debate.Try to look at EP more as a numerical representation of the crowd reaction to the goal.

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