In multiple regression, How to find the range of independent parameter for which regression equation is applicable/valid? I have generated a multiple-regression equation/model from the data.
Parameter A in the data varies from $0.1$ - $10$.
Parameter B in the data varies from $100$ - $1800$.
Parameter C in the data varies from $0.1$ - $0.5$.
The model is,
$y = c + \log A + \frac{1000}{B} + \log C$
Is this regression equation valid outside the range of the above parameters?
Is there any method to find out in what range of A, B and C the above regression model is applicable/valid?

Mathematically, the equation would be valid when $A>0, C>0$ and $B\neq 0$, you can substitute and find some $y$ for any given $A,B,C$ satisfying these conditions.
Estimating outside the ranges is extrapolating and since you do not have data to validate those ranges, it'd be nearly impossible to know if your model does a good job there; except the domain know-how you can apply on the found target values.
