... visualization. Which lines?18.2. ORDINARY LEAST SQUARES 491 15-dimensional space, but already in this 3-dimensional subspace. In other words,[DM93, Figure 1.3] is valid in all dimensions! In the 15-dimensional ... Sometextbooks, such as [DM93], never even give the leftmost term in formula (18.3.6)according to which R2is the sample correlation coefficient. Other textbooks, suchthat [JHG+88] and [Gre97], do give ... orthogonal to ι. See Problem 1 89. Problem 244. In the reggeom visualization, see Problem 350, in which x1isthe vector of ones, which are the vectors Dx2and Dy ? 498 18. MEAN-VARIANCE ANALYSIS...