1) "In regression models, there are two types of variables:
X = independent variable
Y = dependent variable
Y is modeled as random.
X is sometimes modeled as random and sometimes it has fixed value for each observation."
I don't understand the meaning of the last line. When is X random? When is X fixed? Can anyone illustrate each case with a quick example?
note: also under discussion in "Talk Stats Forum" & "SOS math cyberboard"