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Re: st: RE: Interpretation of quadratic terms


From   Michael Norman Mitchell <Michael.Norman.Mitchell@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Interpretation of quadratic terms
Date   Fri, 12 Mar 2010 10:11:26 -0800

Dear Rodolphe

The behavior is a reflection of the problem of collinearity. When "age" is used in the model, the collinearity is not so severe and "margins" is able to produce an estimate. But, when "year" is used in the model, the collinearity is so severe that "margins" is not even able to produce an estimate. That was the point of the example, that sometimes collinearity is more of a problem than other times, and that sometimes centering is necessary and other times centering is not necessary.

  I hope that helps,


Michael N. Mitchell
See the Stata tidbit of the week at...
http://www.MichaelNormanMitchell.com

On 2010-03-12 3.55 AM, Rodolphe Desbordes wrote:
Dear Michael,

In that case, -margins- behaviour is slightly puzzling. Could the absence of output be explained by the fact that the year 1970 did not exist in your sample?



.  sysuse nlsw88, clear
(NLSW, 1988 extract)

.
. generate year = age + 1968

.
end of do-file

. tab year

        year |      Freq.     Percent        Cum.
------------+-----------------------------------
        2002 |         53        2.36        2.36
        2003 |        260       11.58       13.94
        2004 |        257       11.44       25.38
        2005 |        225       10.02       35.40
        2006 |        219        9.75       45.15
        2007 |        234       10.42       55.57
        2008 |        208        9.26       64.83
        2009 |        222        9.88       74.71
        2010 |        160        7.12       81.83
        2011 |        165        7.35       89.18
        2012 |        163        7.26       96.44
        2013 |         78        3.47       99.91
        2014 |          2        0.09      100.00
------------+-----------------------------------
       Total |      2,246      100.00


Best regards,

Rodolphe


________________________________________
From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] On Behalf Of Michael Norman Mitchell [Michael.Norman.Mitchell@gmail.com]
Sent: 12 March 2010 07:33
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: Interpretation of quadratic terms

Dear Rodolphe

    Thanks for your message. I think it is possible that you read a
little bit more into my message than I intended... I was not purporting
to compute marginal effects, and hence did not use the "dydx()" option.
I was just showing how to compute the predicted value of "y" for a given
value of "x". That is another way that the margins command can be used.

Best regards,

Michael N. Mitchell
See the Stata tidbit of the week at...
http://www.MichaelNormanMitchell.com

On 2010-03-11 8.32 AM, Rodolphe Desbordes wrote:
Dear Eric,

I apologise for my lack of clarity regarding the -margins- command. Even though I have hardly used it, exchanges on the Statalist have made it clear that -margins- can do many things, including the calculation of marginal effects. However Michael did not seem to have used the correct syntax; I believe that the option "dydx" must be used to obtain marginal effects.

   Rodolphe

________________________________________
From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] On Behalf Of DE SOUZA Eric [eric.de_souza@coleurope.eu]
Sent: 11 March 2010 15:01
To: 'statalist@hsphsun2.harvard.edu'
Subject: RE: st: RE: Interpretation of quadratic terms

Dear Rodolphe,

I don't have Stata 11 either. But according to the preface to the new edition of Cameron and Trivedi, Microeconometrics using Stata:
"Second, we describe the new margins command for prediction and for computation of
marginal effects in regression models. The margins command with options including the
dydx() option replaces the Stata mfx command and the user-written margeff command.
Additionally, the margins command when used in conjunction with factor variables can
simplify computation of marginal effects in models with interactions. "

So the margins command is Stata 11 does allow one to calculate marginal effects

Eric de Souza
College of Europe
BE-8000 Brugge (Bruges)
Belgium

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Rodolphe Desbordes
Sent: 11 March 2010 14:59
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: RE: Interpretation of quadratic terms

I do not have Stata 11 installed on this computer and I am not vary familiar with the -margins- command. However, using -margins-, you did not calculate the marginal effect but the predicted value of wage, at a given value of age.


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