Re: st: logistic tranformation, proportion variables

[Marck Bulter <177316mb@student.eur.nl>]

Maarten buis wrote:
> --- Marck Bulter <177316mb@student.eur.nl> wrote:
>   
> > Do you think that the conversion to 0.001 is appropriate? 
> >     
> The problem here is to choose an appropriately small value for zero. If
> you choose a value that is too small, the logit transformation will
> result in excessively negative values, leading to outliers. On the
> other hand, you certainly don't want this value to be larger than the
> smallest non-zero value in your data. So this will require some
> experimatation and the end result will to some extend remain
> arbitrairy. (For inspection of normality of the residuals I
> wholehearedly recommend the -hangroot- package downloadable from ssc,
> though I admit that I am not completely unbiased)
>
>   
> > And more important, is it appropriate to use logistic transformed
> > variables both as dependent and independent variables?
> >     
> There is no reason why did would be illegal. I would two things to make
> my choice: 1) have a look at some scatter plots of logit(y) against x,
> and logit(y) against logit(x), 2) estimate a model with logit(x) and a
> model with x and inspect the residuals. 
> Hope this helps,
> Maarten
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology
> Vrije Universiteit Amsterdam
> Boelelaan 1081
> 1081 HV Amsterdam
> The Netherlands
>
> visiting address:
> Buitenveldertselaan 3 (Metropolitan), room Z434
>
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
>
>       __________________________________________________________
> Sent from Yahoo! Mail - a smarter inbox http://uk.mail.yahoo.com
>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>   
Dear Maarten,

In a previous email that I replied a moment ago, I tried the conversions 
but the results are everything but positive, if you have a minute have a 
look.

In short:

- y vs x, resid's are terrible
- logit y vs x , yields residuals that are appalling,
- logit y vs logit x, gives a situation with almost homoskedastic and 
almost normal resid's. Therefore I am some what confident to give this 
model a GO. But still the 0 values are problematic.

Thanks for suggestion for  hangroot, looks good.

Bedankt Maarten

regards,
Marck Bulter
EUR, financial economics





*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/