Linear Regression Models (1)

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?

http://www.geocities.com/asdfasdf23135/stat17.JPG

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