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Given a data set where there exists multiple different observed y-values for a given x-value, how do I calculate Mean Squared Error? The formula implies that I subtract the predicted from a singular observed value, but what if there is multiple?

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    $\begingroup$ What do you mean there are multiple? Are multiple values linked to a single observation or does it just means that for a given $x$ you happened to have multiple datapoints? $\endgroup$
    – Firebug
    Oct 5 at 0:55
  • $\begingroup$ @Firebug For a given x value, there are multiple data points. i.e. (1,2), (1,3), (1, 5), etc. $\endgroup$
    – feonyte
    Oct 5 at 0:57
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Just take the squared error multiple times.

If you have {...,(1,1),(1,2),(1,3),...} and you predict y=2 at x=1, the error sum of squares would be

$...+(1-2)^2+(2-2)^2+(3-1)^2+...$

Then, just divide it by the number of observations to get MSE.

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