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Statistical Analysis Q&A

Discussion in 'NYCFC Supporters Discussion' started by Shwafta, Oct 31, 2018.

  1. Shwafta

    Shwafta Active Member

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    Alright so I figured I'm not the only statistical layman who has questions about how the statistical analysis done by some people on this forum (i.e. dummyrundummyrun and the rest of y'all) is actually calculated, or just don't understand how/why it works. Therefore, a thread dedicated to Statistical Analysis-related things feels appropriate.

    For example, let me start off with a question I've had for a while now:
    There is a lot of talk using xGF and xGA, but what exactly determines how "likely" a chance is? Clearly we saw in an earlier post (by I forget who) showing the xGF/A difference based on who calculates it, so what is the actual determiner? And when determining these, do they also take into account "unlucky" or how well prepared the other team's gk/defense is for a shot of that type?
    For example: Take a shot like that awful Wallace miss from earlier this year. Does the calculation take into account 1) Wallace might flub it by missing/skying the ball; 2) The goalkeeper might be standing directly in front of the ball; etc?
     
    Christopher Jee and dummyrun like this.
  2. dummyrun

    dummyrun Active Member Elite Donor

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    Here are three good places to start on understanding ASA's expected goals model:

    These are kind of dense but should answer your questions if you spend a little time with them. In short: (1) Yes. (2) Sort of (this is what the keeper model is for). Good topic for happy hour.
     
  3. Shwafta

    Shwafta Active Member

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    So for the ASA's model they say they do it based on how far the goalkeeper would have to dive had he placed himself in the optimal location. Now, in MLS is that a statement you can make? I feel like a lot of goalkeepers in the league wouldn't be able to position themselves there.
    [​IMG]
    For example, take this crudely-made model (With like 20% mathematical accuracy for bisection and perpendicularity) based on a cross to Wallace and a classic Wallace-like miss. Normally I'd assume the goalkeeper would be somewhere in the green area (where the black squares are the posts and the black circle is the ball/Wallace) as because of the cross, he would be positioned on the other side of the goal and running back across to where Wallace would be standing with the ball... So wouldn't that increase the xG in actuality? And the converse applies as well.
    Or is there a key point I missed, such as "xG doesn't care about the actual positioning of the keeper, etc."
     

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