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Re: st: RE: RE: what does it mean the default for -mfx- is discrete when it is evaluated at the mean?


From   Nirina F <fstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: RE: what does it mean the default for -mfx- is discrete when it is evaluated at the mean?
Date   Thu, 3 Jun 2010 09:07:32 -0400

Fair enough! Many thanks for the clarification. Best regards, Nirina

On Thu, Jun 3, 2010 at 5:16 AM, Martin Weiss <martin.weiss1@gmx.de> wrote:
>
> <>
>
> You can try all of this yourself. As long as you use default settings (first
> loop below), Stata treats the dummy covariate as a discrete variable, in
> accordance with intuition, and the point at which you evaluate it does not
> change the marginal effects. It is only when you tell it to treat the dummy
> as a continuous variable (second loop) that the marginal effect changes. The
> other covariates are held at their mean throughout, btw.
>
> ***********
> sysuse auto, clear
> gen byte dummy=rep78<4
> logit  foreign gear_ratio turn length dummy
> forv i=.1(.4).9{
> mfx compute, varlist(dummy gear_ratio) dydx at(mean dummy=`i')
> }
> forv i=.1(.4).9{
> mfx compute, varlist(dummy gear_ratio) dydx at(mean dummy=`i') nodisc
> }
> ***********
>
>
> HTH
> Martin
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nirina F
> Sent: Donnerstag, 3. Juni 2010 04:18
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: RE: RE: what does it mean the default for -mfx- is discrete
> when it is evaluated at the mean?
>
> Thank you very much for the responses.
> May be : What you mean is that the rest of the variables are evaluated
> at the mean but the independent dummy variable is evaluated at the
> value 1 (instead of for example the mean at .37)
> Nirina
>
> On Tue, Jun 1, 2010 at 5:39 PM, Martin Weiss <martin.weiss1@gmx.de> wrote:
>>
>> <>
>>
>> This was a knee-jerk reaction of course, since in a linear regression the
>> marginal effects would always be constant.
>>
>> Still, what is the problem with saying that the marginal effect displayed
> is
>> for going from zero to one, i.e. the only move that a dummy can possibly
>> make? The fact that the share of ones in your sample is 37% does not
> change
>> this fact, does it?
>>
>>
>> HTH
>> Martin
>>
>>
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu
>> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Martin Weiss
>> Sent: Dienstag, 1. Juni 2010 23:27
>> To: statalist@hsphsun2.harvard.edu
>> Subject: st: RE: what does it mean the default for -mfx- is discrete when
> it
>> is evaluated at the mean?
>>
>>
>> <>
>>
>> The point of evaluation does not seem to matter at all in this code,
> though:
>>
>>
>> ***********
>> sysuse auto, clear
>> reg price weight length foreign
>> mfx compute, dydx at(mean foreign=0)
>> mfx compute, dydx at(mean foreign=1)
>> mfx compute, dydx at(mean foreign=.2)
>> mfx compute, dydx at(mean foreign=.5)
>> mfx compute, dydx at(mean foreign=.8)
>> ***********
>>
>>
>> HTH
>> Martin
>>
>>
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu
>> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nirina F
>> Sent: Dienstag, 1. Juni 2010 16:09
>> To: statalist@hsphsun2.harvard.edu
>> Subject: st: what does it mean the default for -mfx- is discrete when it
> is
>> evaluated at the mean?
>>
>> Hello,
>> I have a dummy for dependent and main independent variable. I am using
>> ivprobit.
>> When I try to get the marginal effects, mfx  shows you that the
>> default setting is 'discrete', that is, evaluate the marginal effect
>> of a dummy going from 0->1. But it is evaluated at the mean (and under
>> x it shows 0.37 for the independent variable) so I am a little bit
>> confused about the interpretation of the marginal effect.
>> Thank you for your help.
>> Nirina
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