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@wikipedia


A number characterising the prediction quality:

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R^2 = 1 - \frac{MSE(x, \hat x) = \frac{1}{n} \sum_{i=1}^n (x_i - \hat x_i)^2}{MSE(x, \bar x)}

where 

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bodyx = \{ x_1, \, x_2, \, x_3 , ... x_N \}

a variable represented
by 
by a discrete data set of numerical samples

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body\hat x

predictor of variable 

LaTeX Math Inline
bodyx
 

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= \{ \hat x_1, \, \hat x_2, \, \hat x_3 , ... \hat x_N \}

discrete set of numerical samples of variable  

predictor of variable 

LaTeX Math Inline
bodyx
, represented by another discrete data set of numerical samples,

with the same number of samples 

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body

x

N
 predicted at the same conditions as the original samples 

LaTeX Math Inline
body \{

\hat

x_1, \,

\hat

x_2, \,

\hat

x_3 , ...

\hat

x_N \}

discrete set of predictors for the corresponding samples of variable 

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body\bar x

mean value of the variable 

LaTeX Math Inline
bodyx
, which can be considered as some sort of extreme predictor with zero variability

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bodyMSE(x, \hat x)

mean square error between a variable 

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bodyx
 and its predictor 

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body\hat x

LaTeX Math Inline
bodyMSE(x, \bar x)

mean square error between a variable 

LaTeX Math Inline
bodyx
 and its mean value 
LaTeX Math Inline
body\bar x


Thje