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LaTeX Math Block
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R^2 = 1 - \frac{MSD(x, \hat x)}{MSD(x, \bar x)} = 1 - \frac{\sum_i (x_i -\hat x_i)^2}{\sum_i (x_i -\bar x)^2}

where 

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

observed  variable represented by a discrete datasetof numerical samples

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

predictor of variable 

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bodyx
, represented by another discrete dataset of numerical samples,

with the same number of samples 

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bodyN
 predicted at the same conditions as the original samples 
LaTeX Math Inline
body \{ x_1, \, x_2, \, x_3 , ... x_N \}

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body--uriencoded--\bar x = \frac%7B1%7D%7BN%7D \sum_i x_i

mean value of the variable 

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

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bodyMSD(x, \hat x)

mean square deviation between a variable 

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bodyx
 and its predictor 
LaTeX Math Inline
body\hat x

LaTeX Math Inline
bodyMSD(x, \bar x)

mean square deviation between a variable 

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

...

The 

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bodyR^2
  values falling outside the above range indicate a substantial mismatch between variable
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bodyx
 and model prediction 
LaTeX Math Inline
body\hat x
 and have a meaning that gap between predicted and actual values is higher than the variance of the actual data.

See also

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Formal science / Mathematics / Statistics / Statistical Metric