yi∼N(y^i,σ2ε)
y^xi
y^ixiβ^. This leads to the formulation @DikranMarsupial presents:
yi∼N(xiβ^,σ2ε)
It is worth recognizing that this is exactly the same as your first formulation, because both stipulate normal distributions and the expected values are equal. That is:
E[xiβ^]=E[xiβ^+E[N(0,σ2ε)]]=E[xiβ^+0]=E[xiβ^]
(And obviously the variances are equal.) In other words, this is
not a difference in assumptions, but simply a notational difference.
So the question becomes, is there a reason to prefer presenting the idea using the first formulation?
I think the answer is yes for two reasons:
- People often confuse whether the raw data should be normally distributed (i.e., Y), or if the data conditional on X / the errors should be normally distributed (i.e., Y|X / ε), for example, see: What if residuals are normally distributed, but y is not?
- People also often confuse what is supposed to be independent, the raw data or the errors. Moreover, we often mention the fact that something should be iid (independent and identically distributed); if you are thinking in terms of Y|X this can be another potential source of confusion, as Y|X can be independent, but cannot be identically distributed unless the null hypothesis holds (because the mean would vary).
I believe these confustions are more likely using the second formulation than the first.