Is Upper R squared Subscript Upper Y 1.2 the proportion of variation in Upper Y which is explained by the full model, the proportion of variation in Upper Y which is explained by Upper X 2 but not explained by Upper X 1 alone, the proportion of variation in Upper Y which is explained by Upper X 1 but not explained by Upper X 2 alone, or the proportion of variation in Upper Y which is not explained by both Upper X 1 and Upper X 2 alone