R2 is the coefficient of determination. This statistic represents the proportion of error for which the regression accounts.
You can use many methods to calculate R2. Predictor uses the equation:
R2 can be extremely large in cases when the regression constant is omitted, even when the correlation between Y and X is weak. Because it can be meaningless, many applications do not mention this statistic. Predictor provides this statistic but it is not used for stepwise regression when there is no regression constant.