When constructing the matrix X why is it necessary to include the 5th column for age squared? In general, if age is included as an observation, do we always need to add an extra column for age squared and does this apply to other numeric values. Thanks
Hi, For part (ii) of this question, how does df1-df2= 2? from my calcs df2= 4 observations less 3 parameters (age, size, pre existing condition) df1= 4 observations less 2 parameters (age, pre existing condition) Thanks!
Age of dog is being modelled using a quadratic, which has 3 parameters (ie y=ax^2+bx+c). But we don't need to include the constant c because we'll already have a constant in the original model. So by including Age of dog, we'll be using two extra parameters. Hence the change in degrees of freedom is 2.
