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Re: st: Explanatories that vary only over time in panel data


From   Jeffrey Wooldridge <jmwooldridge60@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Explanatories that vary only over time in panel data
Date   Tue, 5 Feb 2013 16:52:31 -0500

Agree there is nothing special that needs to be done, provided you are
still able to assume observations are independent across i. One
drawback is that if you insist on having P_t as an element of X_it
then you cannot include a full set of time period dummies. If you are
not interested in the coefficient on P_t then I would drop it and opt
for a a full set of time dummies -- provided your N is sufficiently
large relative to T.

JW

On Wed, Jan 30, 2013 at 12:17 PM, Aljar Meesters
<aljar.meesters@gmail.com> wrote:
> Estimating a panel regression model where variable(s) do not vary over
> individuals is possible, as long as they vary over time. The same
> holds for adding a time trend to your model. Of course, the
> variable(s) that you want to add should not be collinear with the time
> trend.
> I do not completely understand what you mean with: "If we just repeat
> the price entries for all i districts in our input data we assume that
> Stata will think that there are in fact multiple observations. So the
> standard errors can not be correct.".
> You have these multiple observations per district, so, why should the
> standard errors not be correct? Maybe what worries you is your
> assumption that all prices are equal between all districts. I do not
> know if such an assumption is justified. You can take the average Y
> over districts and regress Y_t = a + b*X_t + c*t instead.
> Best,
>
> Aljar
>
> 2013/1/30 ARDE DE <wsbbarde@gmail.com>:
>> Dear All:
>>
>> Our question is about both, the theory of panel data analysis and
>> (later on) its implementation in Stata.
>>
>> The model we have in mind is:
>> Y_it = a_i + b*X_it + c*t + u_it,
>> i.e. the standard FE/RE framework but with a linear time trend.
>> One of our explanatory variables in X_it does not vary across
>> individuals, it only varies across time (it is a price variable that
>> is assumed to be the same across all districts i within the country we
>> consider).
>> How do we tell Stata about this?
>> If we just repeat the price entries for all i districts in our input
>> data we assume that Stata will think that there are in fact multiple
>> observations. So the standard errors can not be correct.
>> The same question applies to our linear time trend c*t. Can we just
>> include the "year" variable repeatedly (although it does not vary
>> across the index i)?
>>
>> We were not able to find an answer in Wooldridge (2001): Econometric
>> Analysis of Cross Section and Panel Data, nor in Matyas et al. (2008):
>> The Econometrics of Panel Data, 3rd ed.
>>
>> Has someone used a similar model?
>> Any help/hints will be highly appreciated
>>
>> Arde
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