Reading "Econometrics" by Fumio Hayashi, from Princenton University Press, ISBN 0-691-01018-5, in page 13 by "Fixed Regressors" subtitle, it is stated:
"We have presented the classical linear regression model, treating the regressors as random. This is in contrast to the treatment in most textbooks, where X is assumed to be "fixed" or deterministic. If X is fixed, then there is no need to distinguish between conditional distribution of the error term [...] and the unconditional distribution"
I have problems understanding this paragraph. I think it says that if you consider X fixed, then it is "constant" and the conditional distribution of the error term and the unconditional distribution is the same because conditional distribution over a "constant" is equal to unconditional distribution?
On the other hand, I do not know why this is relevant, every time a run an
R I know X, it is always fixed! I am missing something here... may you help me?