University of Michigan - Ann Arbor

R-Squared ($R^2$)—the proportion of variance explained—measures how well a prediction model for "regression" fits the given data. It is independent of the scale of $Y$.
$$0 \leq R^2 \leq 1$$
The larger the R-Squared value, the closer it is to 1, the better the regression model explains the variation of observations around its mean.
$$ R^2 = \frac{TSS-RSS}{TSS} = 1 - \frac{RSS}{TSS} = 1 - \frac{\sum_{i=1}^{n} (y_i - \hat{y}_i)^2}{\sum_{i=1}^{n} (y_i - \bar{y})^2} $$

R-Squared (Coefficient of Determination)

Adjusted R-Squared is a version of R-Squared that is adjusted for the number of predictors (independent variables) in a model. Its advantage over R-Squared is that it accounts for statistical shrinkage.
The normal R-Squared statistic tends to hurt more when more independent variables occur in the system. i.e., we cannot use it to compare models with different numbers of predictors. The adjusted R-squared can be negative, but it’s usually not. It is always lower than the R-squared.
$$\bar{R}^2 = 1 - (1 - R^2)\frac{n - 1}{n - p -1}$$
Where $p$ is the number of predictors.

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