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[no subject]
HTH
Ben
Sent from my iPad
On 1 Feb 2013, at 19:15, nigussie Tefera <[email protected]> wrote:
> Dear statlist users
> Thanks for your all support, finally I could manage to modify poiâ??s nlsurquaids -- demand up date (2008)â??to augment for zero consumption expenditure. In spite of econometric issues raised by Poi and other in our last e-mail conversation, the programme performs well on STATA 12 for up to five food groupings. However, it takes hours and hours if there are larger food groupings. I want to use for 10 food groupings, in my case, and couldnâ??t get the results yet (have been waiting for about 5 hours now). I wonder if there is ways (means) to speed up iteration a bit faster. Any suggestion is highly appreciated.
> Best
> Nigussie
>
> ----- Original Message -----
> From: Michael Musyoka <[email protected]>
> To: "[email protected]" <[email protected]>
> Cc:
> Sent: Thursday, January 31, 2013 3:56 PM
> Subject: Re: st: QUAIDS
>
> Ben,
> Yes, that is a key issue in demand system identification, that is, making the exclusion restriction. The variables selected should follow the logical characteristics of an instrumental variables. The first stage-probit estimation of the system, is identified by inclusion of the equation's respective PDFs. Identification of the demand equations in the second stage is always a challenge as is with Heckman two Step Models. You would usually look for a variable that explains probability of
> purchases (usually referred to as market participation and computed as a
> dummy variable from the budgets shares) but not demand in the second stage. More often than not, you would follow logic, literature and/or theory in identifying such a variable. Perhaps, you may want to have a look at the book by: Cameron and Trivedi 2005. Microeconometric Methods and Applications-Chapter 16 on Tobit and Selection Models, just to give you an idea.
>
> Musyoka,
> [email protected]
>
>
>
>
> ----- Original Message -----
> From: ben jones <[email protected]>
> To: "[email protected]" <[email protected]>
> Cc:
> Sent: Thursday, January 31, 2013 12:34 PM
> Subject: Re: st: QUAIDS
>
> Dear All
>
> Thanks for everyone's contributions, I have been following this exchange with interest. On a practical note might I ask those of you who have estimated this type of system, what sort of variables you have tended to use in the first stage probit which are not included in the conditional demand equation (I have struggled to successfully make this exclusion)?
>
> Many thanks,
>
> Ben
> [email protected]
>
>
> ________________________________
> From: nigussie Tefera <[email protected]>
> To: "[email protected]" <[email protected]>
> Sent: Thursday, 31 January 2013, 6:03
> Subject: Re: st: QUAIDS
>
> Thanks a lot Murat,
>
> Nigussie
> ----- Original Message -----
> From: Michael Musyoka <[email protected]>
> To: "[email protected]" <[email protected]>
> Cc:
> Sent: Thursday, January 31, 2013 8:45 AM
> Subject: Re: st: QUAIDS
>
> Murat,
> Yes the budget shares are actually not scaled by use of the CDF....
>
> More published literature include
>
> Two-Step Estimation of a Censored System of EquationsAuthor(s): J. Scott Shonkwiler and Steven T. Yen Reviewed work(s):Source: American Journal of Agricultural Economics, Vol. 81, No. 4 (Nov., 1999), pp. 972-982
>
>
> Su, Shew-Jiuan B. and Yen, Steven T.(2000) 'A censored system of cigarette and alcohol consumption',
> Applied Economics, 32: 6, 729 â?? 737...
>
> Quasi- and Simulated-Likelihood Approaches to Censored Demand Systems: Food Consumption by Food Stamp Recipients in the United States
> Author(s): Steven T. Yen, Biing-Hwan Lin, David M. Smallwood
> Source: American Journal of Agricultural Economics, Vol. 85, No. 2 (May, 2003), pp. 458-478
>
> Impacts of world prices transmission to domestic rice markets in rural Iran
> M. Bakhshoodeh; Food Policy 35 (2010) 12â??19
>
>
>
>
> ----- Original Message -----
> From: nigussie Tefera <[email protected]>
> To: "[email protected]" <[email protected]>
> Cc:
> Sent: Tuesday, January 29, 2013 5:57 PM
> Subject: Re: st: QUAIDS
>
> Dear Murat,
>
> Thanks a lot for your notice. I was completly wrong in saying so but I corrected that one in my latter e-mail.
>
> Best
>
> Nigussie
>
> ----- Original Message -----
> From: Murat Genc <[email protected]>
> To: "'[email protected]'" <[email protected]>
> Cc:
> Sent: Tuesday, January 29, 2013 3:12 PM
> Subject: RE: st: QUAIDS
>
> Dear Nigussie,
>
> I have never seen a suggestion that left hand size variables (the budget share) should be multiplied by CDF.
>
> You may want to have a look at Lambert at al. (2006) "Fish and Meat Demand in Canada: Regional Differences and Weak Separability", Agribusiness Vol 22 (2) 175-199 for a good application of Quaids with Shonkwiler and Yen correction. A more recent application is Ecker and Qaim (2011) "Analyzing nutritional impacts of Policies: An empirical study for Malawi," World Development Vol 39 (3) 412-428.
>
> Murat
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of nigussie Tefera
> Sent: Wednesday, 30 January 2013 12:33 a.m.
> To: [email protected]
> Subject: Re: st: QUAIDS
>
> Dear Musyoka,
>
> Both both the righ and left hand size variables should have to be multiply with CDF. In fact, the PDF variables should also be added to both right and left hand side variables. In the left hand side we would expect equation like this "wi*CDFi + hiPFDi" instead of only "wi", where wi the budget share of commodity i. The CDFi and PDFi is expected to be derived from multivariate probit analysis following suggestion of Yen, Lin, and Smallwood (2003).
>
> Because of such complexity, it is difficult for me to estimate Quaids agumented for zero consumption expeniture with CDF and PDF. I badly need suggestions from STATA userlist, partiuclary who have experienced and solved such problem. I am using Ethiopian Rural Household Survey (ERHS) panel data. Zero consumption expenditure is very server for the ten food groups I considerd for demand analysis. These include teff, barely, wheat, maize, sorghum, pluses, root crops, fruits & vegetables, animal products and other food. The first five items are the major cereals and staples in Ethiopian dish.
>
> Best
>
> Nigussie
>
>
> ----- Original Message -----
> From: Michael Musyoka <[email protected]>
> To: "[email protected]" <[email protected]>
> Cc:
> Sent: Tuesday, January 29, 2013 2:02 PM
> Subject: Re: st: QUAIDS
>
>
> Brian,
> I think you should check on whether it is ONLY the left hand side (dependent variables) which are the shares are multiplied with the CDF variable. I need to be corrected if my presumption that only the right hand side variables are multiplied by CDF is wrong.
> Again, Brian, while I had you had exchanged some emails on nlsur QAIDs previously on the issue of zero budget shares, I had to do the zero budget shares correction manually as you suggest below. The results seem plausible though I did not drop one equation.
>
>
>
> Musyoka,
>
>
>
>
> ________________________________
> From: Brian P. Poi <[email protected]>
> To: [email protected]
> Sent: Monday, January 28, 2013 5:25 PM
> Subject: Re: st: QUAIDS
>
> On 01/27/2013 05:48 AM, nigussie Tefera wrote:
>> Dear Murat and Kolawole,
>>
>> Thanks a lot for your the help!
>>
>> As both of you rightly said, I am planning to multiply the CFD variables by all variables and it is only PDF variables that is going to be additional variables in the estimation techniques. The "nlsurquaidsNNP" program that I tried to write did exactly in the same way. The CDF is multiplied with the all variables and the PDF is the additional variables to the system. In my first post, I didn't spell it out properly, I am sorry for that. In fact, Shonkwiler and Yen (1999) warned that the share of expenditure will not be added up to one, as you also mentioned, and they suggest running full set of equations rather than for N-1 equations. However, Yen, Lin, and Smallwood (2003) as cited in Zheng and Henneberry (2010) suggested to recalculate share of one of the food items under study in such a way that the adding property to be hold. i.e. if Si=wi+Ï?PDFi . where si is the new budget share and wi the original budget share and PDF is the probability density
>> function derived from the first-stage multivariate probit function. Then they indicated the Sn=1- . I want to apply the same approach and exclude one of the food items in estimating the demand equations so as to control for singularity. And also as both of you mentioned correcting for the standard error is one of the difficulties in using this approaches. I think there are research works on progress to correct for it. In estimating the elasticities, again, we need to take into accounts the CDF and PDF variables and hence the standard formula for elasticity estimate doesnâ??t apply once the system is controlled for CDF and PDF variables.
>>
>> I prefer to use the recently released Boiâ??s quaids stata program to estimate the system (may be with modification) but I am not yet sure whether the program is already controlled for observed zero consumption expenditure as suggested by Shonkwiler and Yen(1999). Dear Dr. Poi, I need your suggestion in this regard.
>>
>> Best
>> Nigussie
>>
>
> Nigussie,
>
> Right now the -quaids- program recently published in the Stata Journal
> (Poi, 2012) does not do anything with respect to the
> zero-expenditure-share issue.
>
> Moreover, -quaids- handles demographic variables using an extension of the
> method introduced in Ray (1983). My very strong suspicion is that
> specifying a CDF, PDF, or inverse Mills ratio in the demographics() option
> of -quaids- as if such a variable were just another demographic like the
> number of children or the number of adults in a household is _not_ the
> right thing to do.
>
> I often get asked about Stata code for demand system estimation with
> zero-expenditure shares. I have nothing to show for that as of now. As
> you know, there have been many different methods proposed in the
> literature. I have never actually implemented any of them myself.
>
> -- Brian Poi
> -- [email protected]
>
> References
> ==========
> Poi, B. P. (2012). Easy demand-system estimation with quaids. Stata
> Journal, 12, 433--446.
>
> Ray, R. (1983). Measuring the costs of children: an alternative approach.
> Journal of Public Economics, 22, 89--102.
>
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