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Re: st: margins with logarithmic offsets
From
Joerg Luedicke <[email protected]>
To
[email protected]
Subject
Re: st: margins with logarithmic offsets
Date
Sun, 11 Aug 2013 10:19:19 -0400
You could constrain the coefficient of the logged offset variable
yourself in which case -margins- can access it:
*--------------------------------------------------------
webuse dollhill3, clear
gen ln_pyears = ln(pyears)
constrain define 1 _b[ln_pyears] = 1
poisson deaths i.smokes i.agecat ln_pyears, constraint(1)
margins, dydx(smokes) at(ln_pyears == `=ln(20000)')
*--------------------------------------------------------
Joerg
On Fri, Aug 9, 2013 at 6:19 PM, Dimitriy V. Masterov <[email protected]> wrote:
> I have two questions about evaluating marginal effects at a specific
> value of the logarithmic offset variable. Adding the at option to the
> margins command gives offset not found in list of covariates error.
>
> I did not find anything on this issue in the documentation, but there
> was an old Stata FAQ about the mfx (an ancestor of margins) that
> suggested merely replacing like this:
>
> webuse dollhill3, clear
> poisson deaths i.smokes i.agecat, exposure(pyears)
> margins, dydx(smokes)
> replace pyears =20000
> margins, dydx(smokes)
>
> 1) Is there a less clunky way to do this?
> 2) Does this correspond to -margins, dydx(smokes) at(pyears ==20000)-?
>
> Stata mfx FAQ link:
> http://www.stata.com/support/faqs/statistics/marginal-effects-after-offset/
> *
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