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Feb 7, 2018 at 21:44 comment added Ben Okay, in that case I think I may have misinterpreted your question. Hopefully some of what I've written is helpful to you anyway.
Feb 7, 2018 at 13:44 comment added Olivier My question isn't assuming that $\omega$ has no relationship with $Y$ ($\omega$ is a random variable and, in fact, the only source of randomness relevant in my problem).
Feb 7, 2018 at 4:19 comment added Ben If your question is assuming that $\omega$ has no relationship with $Y$ then you are effectively assuming that $\beta_\omega = 0$. Under this condition the equation for the new model is $\boldsymbol{Y} = \boldsymbol{X} \beta + \boldsymbol{\omega} \beta_\omega + \varepsilon = X \beta + \varepsilon$. Hence, although the new variable changes the design matrix, this does not affect the underlying model form.
Feb 7, 2018 at 3:40 comment added Olivier While the noise variables have no direct relationship with $Y$, they are covariates and hence adding them changes the dimensions of the design matrix (as well as the dimension of the parameters). The equation $Y = X\beta + \varepsilon$ changes with the model.
Feb 7, 2018 at 3:27 comment added Ben Not sure if I interpreted your question correctly. I assume you're saying that the newly added noise variable does not have any relationship with Y? Is that correct? If so, then it would not appear in the model equation, so both models would have the same underlying form.
Feb 7, 2018 at 2:26 comment added Olivier What you mean by "both model have the same underlying equation"? I don't see why this is the case and how you conclude.
Feb 6, 2018 at 6:02 history answered Ben CC BY-SA 3.0