I have a question about some assumptions in multiple regression. Based on the theory in my university we have been taught that before running regression we have to analyze the following assumptions:
- Simple random sampling ( Pretty clear)
- Independency ( pretty clear)
- Trustworthiness of data (pretty clear)
- Errors are normally distributed for every condition of X
- Error means are equal to 0 for every condition of x
- error std.dev is constant for every condition of X
- Probably something i forgot
- Variation in x
There's nothing written in any of the materials/books provided by university about assumption of variation in x. I have not found anything by googling it. Our T.A. would always mention it, but he would go through it so fast that noone even understood what it is.
Anyone could explain what assumption of ''variation in x'' could be relevant for multiple regression ?