To account for Heteroscedasticity, you specify the variance as a function of variables. Since you did not specify the link function but only NOCONST, the exp was used by default. That is, the expectation of the variance in the model = c * exp(x’H ). The parameters of _H. are the elements of H vector. X is a vector you defined at HETERO statement, that is, r1 r3 r5 r7. Clearly, the variance is depending on the r1…, etc. that is why you call that hetero-.
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