4 . Hastie and Tibshirani ( 19 90 ) described a study to determine risk factors for kypho - sis , severe forward flexion of the spine following corrective spinal surgery . The age in months at the time of the operation for the 18 subjects for whom kyphosis was present were 12 , 15 , 42 , 52 , 59, 73, 82 , 91 , 96 , 105, 114 , 120, 121 , 128, 130 , 139, 139 , 157 and for 22 of the subjects for whom kyphosis was absent were 1 , 1 , 2 , 8 , 1 1 , 18 , 22 , 31 , 37 , 61 , 72 , 81 , 97 , 112 , 118 , 127 , 131 , 140 , 151 , 159 , 177 , 206 . ( a ) Fit a logistic regression model using age as a predictor of whether kyphosis is present . Test whether age has a significant effect . ( b ) Boxplot the data at two levels of kyphosis . Fit the model logit ( IT ( 20 ) ) = at Brac + Bzack . Test the significance of the squared age term . Plot the fit , and interpret