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From |
"Dimitriy V. Masterov" <[email protected]> |

To |
Statalist <[email protected]> |

Subject |
Re: st: puzzling semi-elasticity from margins |

Date |
Thu, 16 Jan 2014 11:53:41 -0800 |

Austin, That makes perfect sense. Thanks once again! DVM On Thu, Jan 16, 2014 at 11:41 AM, Austin Nichols <[email protected]> wrote: > Dimitriy V. Masterov <[email protected]>: > You can't really assess dy/dx because your regression is not a model > of how y changes with x but how the conditional mean of y changes with > x. So you can assess > dE[y|x]/dx > assuming you have a good model of the conditional mean in your > regression. Then the semielasticity eydx would be > dE[y|x]/dx * (1/E[y|x]). > > Make sense? > > On Thu, Jan 16, 2014 at 2:01 PM, Dimitriy V. Masterov > <[email protected]> wrote: >> I was under the impression that eydx corresponds to (dy/dx)*(1/y). >> There's an example of on p. 1169 of the Stata 13 manual where for obs >> 5, dy/dx = 0.5, y = 15, and x = 30, so eydx is 0.0333333. My >> assumption was that margins with this option would calculate the >> average over all the observations. That does not seem to be the case >> in this simple example: >> >> sysuse auto >> reg price mpg >> margins, eydx(*) >> gen double me = _b[mpg]*(1/price) >> sum me >> predict phat, xb >> gen double me2 = _b[mpg]*(1/phat) >> sum me2 >> >> margins, eydx gives -.0421593. >> sum me gives -.0451027 >> sum me2 yields -.0421593 >> >> The version where I divide by the predicted price rather than the >> actual price for each observation seems to match the margins output. >> What am I missing here? > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/faqs/resources/statalist-faq/ > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**References**:**st: puzzling semi-elasticity from margins***From:*"Dimitriy V. Masterov" <[email protected]>

**Re: st: puzzling semi-elasticity from margins***From:*Austin Nichols <[email protected]>

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