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06-22-2009 05:30 PM

I have data with 3 variables X,Y,Z and i want to fit a model like ...

Y= a(x) + b(z)+c,

X and Z may have some powers (n) also.How to do it? Please guide

Y= a(x) + b(z)+c,

X and Z may have some powers (n) also.How to do it? Please guide

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06-26-2009 01:51 PM

Your problem specification is incomplete. Let me respond by first asking a few questions.

Why do you think you should use powers of X and Z? Are those powers prespecified? Are X and Z nonnegative? If the powers are not prespecified integer values, then you could run into problems if the algorithm which is used to fit the model tries to raise a negative value to a noninteger power.

Do you want to consider polynomial functions of X and Z? The model below is representative of a polynomial model:

Y = a1(**X**) + a2(**X**2**) + a3**(X**3**) + b1(**Z**) + b2(**Z**2**) + c

Finally (for now, anyway), what motivates the model that you want to fit?

Why do you think you should use powers of X and Z? Are those powers prespecified? Are X and Z nonnegative? If the powers are not prespecified integer values, then you could run into problems if the algorithm which is used to fit the model tries to raise a negative value to a noninteger power.

Do you want to consider polynomial functions of X and Z? The model below is representative of a polynomial model:

Y = a1(

Finally (for now, anyway), what motivates the model that you want to fit?

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06-27-2009 06:07 PM

i have data with one variable Y that changes depending on X and Z. But this change is not linear. I want to find the best fit curve and equation.

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06-27-2009 06:09 PM

The values of X, Y and Z involve some negative values also and secondly the values are like 0.9,... -0.04,... 1.05