A vision scientist obtained a random sample of 88 individuals from a psychology department participant pool. From each participant, he obtained measures of attentional focus (x1), digit span (x2), list-learning efficiency (x3), motion aftereffect duration (x4), and time to complete a trail-making task (y). The sum of squares for y was 128. She submitted these results to a hierarchical regression with x1 and x3 entered on step 1, then x2 and x4 added on step 2. The sum of squares for the ŷ values was 32 on step 1, and 64 on step 2. Therefore, the change in explained variance (ΔR2) is ______.
0.09
0.27
0.25
0.50
