QLIM Procedure
HETERO Statement
HETERO dependent variables ~ exogenous variables </ options > ;
The HETERO statement specifies variables that are related to the heteroscedasticity of the residuals and the way these variables are used to model the error variance. The heteroscedastic regression model supported by PROC QLIM is
For more information about the specification of functional forms, see the section Heteroscedasticity. You can specify the following options after a slash (/):
- LINK=EXP | LINEAR
-
specifies the functional form. You can specify the following values:
- EXP
-
specifies the exponential link function,
- LINEAR
-
specifies the linear link function,
By default, LINK=EXP.
- NOCONST
-
specifies that there be no constant in the exponential heteroscedasticity model.
- SQUARE
-
estimates the model by using the square of linear heteroscedasticity function. For example, you can specify the following heteroscedasticity function:
model y = x1 x2 / discrete; hetero y ~ z1 / link=linear square;The option SQUARE does not apply to exponential heteroscedasticity function because the square of an exponential function of
is the same as the exponential of 
. Hence the only difference is that all 
estimates are divided by two.
You can use the HETERO statement within a Bayesian framework, but you should do this carefully because convergence can be slower than in the homoscedastic case. For more information, see the section Priors for Heteroscedastic Models.