# Re: st: Sample selection?

 From "Suryadipta Roy" <[email protected]> To [email protected] Subject Re: st: Sample selection? Date Sat, 12 Jan 2008 15:06:51 -0600

```Maarten,

Thanks,

On 1/12/08, Maarten buis <[email protected]> wrote:
> --- Suryadipta Roy <[email protected]> wrote:
> > My dependent variable is continuous taking negatitve to positive
> > values. In order to convert it into logarithms, I used a positive
> > transformation and then ran the regression with the logarithm value
> > as the dependent varible. I am finding that the values of the
> > coefficients of my explanatory variables are different from the one
> > when the original value was used as the dependent variable. I was
> > wondering if I need to address sample selection issues when I am
> > using the logarithm and thereby constraining the variable to be
> > positive only, and why the values of the coefficients differ in the
> > two cases.
>
> No, say your original depedent variable is in euros, than your
> coefficient tells you the expected increase or decrease in euros for a
> unit increase in your dependent variable. In your log transformed model
> you get the expected increase or decrease in log(euros) for a unit
> increase in your depedent variable. So the coefficients are different
> because they mean something different.
>
> > Also, my main explanatory variable is categorical in nature ( 0 -
> > 10). Is it alright to treat this as a continuous variable in the
> > regression?
>
> Depends, if it is truely categorical (e.g christian, muslim, hindu,
> etc.) than you can add it as a continuous variable (in the sense that
> Stata won't complain) but the outcome is complete rubbish. If it is
> oridinal than you are assuming that the distance between the categories
> are the same. I you think (and can convince others) that that is ok,
> than you can do it.
>
> Hope this helps,
> Maarten
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology
> Vrije Universiteit Amsterdam
> Boelelaan 1081
> 1081 HV Amsterdam
> The Netherlands
>
> Buitenveldertselaan 3 (Metropolitan), room Z434
>
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
>
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```