x1=[7 1 11 11 7 11 3 1 2 21 1 11 10]';
x2=[26 29 56 31 52 55 71 31 54 47 40 66 68]';
x3=[6 15 8 8 6 9 17 22 18 4 23 9 8]';
x4=[60 52 20 47 33 22 6 44 22 26 34 12 12]';
y=[78.5 74.3 104.3 87.6 95.9 109.2 102.7 72.5 93.1 115.9 83.8 113.3 109.4]';
x=[x1 x2 x3 x4];

stepwise(x,y)