I'm attempting to solve a multiple regression. My problem is that I want the resultant function to be always positive. I need a regression of the machining cutting force for several values of cutting parameters (cutting speed, depth of cut, ...). The cutting force has to be always positive, but with my limited set of parameters (i don't have ALL the possibile speeds or all the possible depths) the resultant function for the force isn't always positive. For exaple: in my experiments I have the tool diameter varying from 15 to 70. If I use my regression and try to calculate the cutting force, for example, with diameter 10 I get a negative value. That's unacceptable, the cutting force has to be positive. I need to constrain the regression so that the function is always positive with the coefficients found and for ANY value of cutting parameters (or at least for positive values of cutting parameters, cause negative values of diameter, depth of cut, ... don't exist too). Some coefficients has to be positive and others negative, but the final function has to be always positive for positive cutting parameters values. How can I do this? I use Matlab but any program would be great if it solves my problem.