st: normality and heteroskedasticity in truncreg

[Anita <anita.metzger@vtxmail.ch>]

Dear Statalist Members
> I am estimating a truncated regression with truncreg. My y-Variable is 
> truncated (400 of 1700 observations) and highly skewed to the right, 
> so I transformed using the logarithm.
> Now I would like to test for Heteroskedasticity and Normality. I did 
> not find any tests.
>
> I tried to analize the residuals graphically by
>
> predict, xb
> gen residual=y-xb, where y ist my observation and xb is the fitted value.
>
> By plotting the residuals against the fitted values, I found a graph 
> with a slope of -1, above this, there are also some observations 
> (graph ist attached).
> How can I interpret this, or is there a better way to interpret 
> truncated regression residuals for heteroscedasticity? Why are there 
> so many negative fitted values? May be I made a mistake calculating 
> the residuals
>
> I also tried to follow the graphical analysis for normal regression 
> for Normality using qnorm and pnorm, but i am not shure if this is 
> appropriate, as this is not a linear estimation.
>
> Can anyone help me please?
> I am very sorry, if this is a silly question
>
> Anita