RE: st: Re: st: Re: st: Re: st: Re: st: Re: st: RE: Truncated sample or Heckman selection‏

[Ebru Ozturk <ebru_0512@hotmail.com>]

Thank you very much for your help. I just would like to make sure that I understand you correctly. Also I am here to learn so please forgive me for asking many questions.

1. I restrict sample to innovating firms and the dependent variables that I am interested in are percentages therefore, you agree on that I should use fractional logit model.

2. I do not need to do Heckman sample selection even though I restrict my sample. I also want to mention that yes I do not have some Xs but I still do have other Xs for non-innovators which I use them as control variables.

3. Do you think using a different model puts me trouble?

4. I use Stata 10 so can I use fractional logit model? 

5. What about if I have a panel data? Is fractional logit model still applicable?

Ebru

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> Date: Fri, 5 Oct 2012 16:21:40 +0100
> Subject: st: Re: st: Re: st: Re: st: Re: st: Re: st: RE: Truncated sample or Heckman selection‏
> From: njcoxstata@gmail.com
> To: statalist@hsphsun2.harvard.edu
>
> Isn't this the same question?
>
> It seems to me that you have two issues, which are independent of each other.
>
> 1. Your response is bounded [0,1] or [0,100], dependent on units.
>
> Joerg, Maarten and I all seem to agree that this is not censoring in
> any strong sense, as values outside the bounds are impossible in
> principle. Thus we don't think that tobit makes sense here.
>
> Daniel's argument appears to be that there is a important sense in
> which this can be considered as censoring. Thus he seems more open to
> the idea that tobit makes sense.
>
> This is not an election and what is right is not determined by
> counting votes. As I don't understand Daniel's argument I certainly
> can't refute it.
>
> Personally I pay little attention to what's popular in the literature
> if I think it's wrong. It's good news for me: I have a chance to try
> to improve practice.
>
> 2. There is a question of what determines which firms do and which do
> not innovate. This is interesting and important and you should model
> it _if you have data to do so_. In your earliest posts in this thread,
> you seemed (to me at least) to imply that you have no other data on
> non-innovating firms with which to model selection, but you should
> know.
>
> This is just my summary. The others who have contributed to the thread
> may want to correct or complement it. Other people of course may have
> other views.
>
> If you are expecting that Statalist can act as a collective oracle
> that can tell you the _correct_ thing to do in your project, that is I
> suggest more than the list can deliver.
>
> Nick
>
> On Fri, Oct 5, 2012 at 3:29 PM, Ebru Ozturk <ebru_0512@hotmail.com> wrote:
> > My question was whether I should use Heckman sample selection as I restricted my sample to innovating firms only.
> >
> > After discussing with you, I also confused as I said before many papers used tobit regression and did Heckman correction and never seen fractional logit model in these papers such as Grimpe & Kaiser, (2010); Cassiman & Veugelers, (2006).
> >
> > These papers are saying that, the data is left censored as it includes many zeros. Also when I search for fractional logit model it says it is bounded between 0 and 1 but my dependent variable includes for instance, 0, 11, 13, 15, 25 values and so on.
> >
> > Maybe I am not very good at statistics but when I read papers they use tobit regression. Whey then?
> >
> > Ebru
> >
> > ----------------------------------------
> >> Date: Fri, 5 Oct 2012 15:02:22 +0200
> >> Subject: st: Re: st: Re: st: Re: st: Re: st: RE: Truncated sample or Heckman selection‏
> >> From: maartenlbuis@gmail.com
> >> To: statalist@hsphsun2.harvard.edu
> >>
> >> On Fri, Oct 5, 2012 at 1:53 PM, Ebru Ozturk <ebru_0512@hotmail.com> wrote:
> >> > I think there is a misunderstanding here. Yes, I have valid and meaningful values on my explanatory variables but when I restrict the sample to innovating firms. By restricting the sample I just exclude the firms that no activity toward innovation at all which I am not interested in them. So, in the restricted sample I can also observe Xs for 0s as my dependent variables are radical and incremental innovation.
> >> >
> >> > I hope it is clear now.
> >>
> >> Now I really don't understand your problem: Is it that you have a
> >> dependent variable that is a proportion and some of these proportions
> >> are 0? Than just use a fractional logit model (just -glm- with the
> >> -link(logit)- and the -vce(robust)- options). For such problems
> >> neither -tobit- nor -heckman- are appropriate nor is there any
> >> truncation going on.
> >>
>
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