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 asrandomand sometimes it hasfixedvalue 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?

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note: also under discussion in "Talk Stats Forum" & "SOS math cyberboard"