Re: Other forums [was: RE: st: RE: how to force same excluded groups across regression models]

[Megan Sheahan <megan.sheahan@gmail.com>]

Richard - I think you pointed me in the correct direction. Thanks!
However, it seems to work for -regress-, but not when I regress using
-xtreg-. Any thoughts on why that might be? There is no help in the
manual.

Fernando - Thanks for your suggestion. Stata still does strange things
when I create separate dummies for each category and interact them
separately.

Megan



On Wed, Oct 12, 2011 at 11:54 AM, Fernando Rios Avila
<f.rios.a@gmail.com> wrote:
> Hi Megan,
> I know this is a more simple solution, but perhaps you should try to
> create the dummies and interaction terms before the different
> specifications are estimated, so you can have more control on which
> variable is used as baseline reference.
> regards,
> Fernando
>
> On Wed, Oct 12, 2011 at 2:38 PM, Megan Sheahan <megan.sheahan@gmail.com> wrote:
>> Thanks Nick. I'll try my question again on here.
>>
>> I am attempting to run the same model as pooled OLS, fixed effects,
>> and CRE. There are a number of interaction terms in my model with
>> categorical variables. When Stata runs these three models, it chooses
>> different groups to exclude in the FE model than in the OLS and CRE
>> models, making the output difficult to compare. For example, I am
>> interacting three zone groups and four soil groups with N. The CRE and
>> OLS models keep all three zones and omit the fourth soil group. The FE
>> model omits the first zone and keeps all four soil groups. Is there
>> any way to tell Stata to exclude the same groups across regression
>> models? I tried the following command, but it did not fix the problem:
>> char varname[omit].
>>
>> For reference, here is the syntax for the three models:
>>
>> /* POOLED OLS */ regress yield_ha c.N_ha#zone_comb
>> c.N_ha#c.N_ha#zone_comb c.N_ha#c.P_ha#zone_comb
>> c.N_ha#soil_group_megan c.N_ha#c.rain_stress#zone_comb seed_ha
>> c.seed_ha#c.seed_ha hectares c.hectares#c.hectares masset_ha
>> c.masset_ha#c.masset_ha rain_stress i.manure i.hybrid
>> c.rain_stress#hybrid i.legume i.crop_per_field i.dist i.FAO_class
>> i.year if prod_func == 1, cluster(hhid)
>>
>> /* FE */ xtreg yield_ha c.N_ha#zone_comb c.N_ha#c.N_ha#zone_comb
>> c.N_ha#c.P_ha#zone_comb c.N_ha#soil_group_megan
>> c.N_ha#c.rain_stress#zone_comb seed_ha c.seed_ha#c.seed_ha hectares
>> c.hectares#c.hectares masset_ha c.masset_ha#c.masset_ha rain_stress
>> i.manure i.hybrid c.rain_stress#hybrid i.legume i.crop_per_field
>> i.dist i.FAO_class i.year if prod_func == 1, fe i(hhid) cluster(hhid)
>>
>> /* CRE, WITH POOLED OLS STRUCTURE */ regress yield_ha c.N_ha#zone_comb
>> c.N_ha#c.N_ha#zone_comb c.N_ha#c.P_ha#zone_comb
>> c.N_ha#soil_group_megan c.N_ha#c.rain_stress#zone_comb seed_ha
>> c.seed_ha#c.seed_ha hectares c.hectares#c.hectares masset_ha
>> c.masset_ha#c.masset_ha rain_stress i.manure i.hybrid
>> c.rain_stress#hybrid i.legume i.crop_per_field i.dist i.FAO_class
>> i.year mN_ha mP_ha mseed_ha mhectares mrain_stress mmanure mhybrid
>> mlegume mcrop_per_field if prod_func == 1, cluster(hhid)
>>
>> Many thanks in advance for your suggestions.
>>
>> Megan
>>
>>
>> On Wed, Oct 12, 2011 at 11:22 AM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
>>> Stata tech support, if it falls within their remit?
>>>
>>> The main other forum I know of is Stack overflow. There is much less Stata traffic there than on this list.
>>>
>>> StataCorp is now on Facebook and Twitter but neither seems well suited to detailed technical questions or answers.
>>>
>>> Without bias, and without prejudice of any kind, I'd say that there are more, and more active, experts on this list...
>>>
>>> But what is it that can't be answered in this forum?
>>>
>>> Very broadly, and without reference to your thread, three main kinds of questions have low probabilities of getting detailed answers on this list. These are expressed in their limiting forms:
>>>
>>> * How should I analyse my data?
>>>
>>> * I am doing X. Is this correct?
>>>
>>> * I am new to Stata. Please give me a personal tutorial on A and B.
>>>
>>> And I don't think other lists are, in practice, more reactive to such questions.
>>>
>>> Nick
>>> n.j.cox@durham.ac.uk
>>>
>>> Megan Sheahan
>>>
>>> Thanks again, Dave. For the broader audience, do you happen to know
>>> where else I can submit a Stata question if it can't be answered
>>> through this listserv?
>>>
>>>
>>> *
>>> *   For searches and help try:
>>> *   http://www.stata.com/help.cgi?search
>>> *   http://www.stata.com/support/statalist/faq
>>> *   http://www.ats.ucla.edu/stat/stata/
>>>
>>
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/statalist/faq
>> *   http://www.ats.ucla.edu/stat/stata/
>>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/