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Assumptions made

The assumption we have made here can be summarized as (and this is going to be useful when we discuss the bias-variance trade off) that there exists a function f(\boldsymbol{x}) and a normal distributed error \boldsymbol{\varepsilon}\sim \mathcal{N}(0, \sigma^2) which describe our data

\boldsymbol{y} = f(\boldsymbol{x})+\boldsymbol{\varepsilon}

We approximate this function with our model from the solution of the linear regression equations, that is our function f is approximated by \boldsymbol{\tilde{y}} where we want to minimize (\boldsymbol{y}-\boldsymbol{\tilde{y}})^2 , our MSE, with

\boldsymbol{\tilde{y}} = \boldsymbol{X}\boldsymbol{\beta}.