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From |
Maarten Buis <maartenlbuis@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: OLS assumptions not met: transformation, gls, or glm as solutions? |

Date |
Wed, 2 Jan 2013 12:05:17 +0100 |

On Fri, Dec 21, 2012 at 1:33 AM, Alan Acock <acock@me.com> wrote: > If I run > > regress qual_p conf_p i.sexrare ston_p forg_p sacr_p > > were all variables but for sexrare are proportion of the maximum possible value, the interpretations are simple. A change in conf_p of one percentage point predicts a xx(coefficient) percentage point change in the outcome. > > When I run > > glm qual_p conf_p i.sexrare ston_p forg_p sacr_p, /// > family(binomial) link(logit) vce(robust) > > is there a clear interpretation of the coefficient or some transformation of the coefficients? > > I'm think the answer should be obvious to me, but it is not. The exponentiated coefficients have an interpretation that in the helpfile of -betafit- (available from SSC) we called relative proportion ratios. In that helpfile we also included an explanation of it. In the example below I apply this to the fractional logit. It is easiest to start with the baseline relative proportion: For a city governed by a majority left wing government with a fairly average priced houses (a proxy for wealth) and population density (a proxy for urbanicity) we expect the city to spent about 11 cents on governing itself (wages of civil servants, maintaining office space for them, etc.) for every euro spent on everything else. This ratio decreases by (1-.85)*100%=-15% if the city is governed by a majority right government, but is virtually the same as the ratio for a completely right government. The ratio of expenditure on government and expenditure on everything else tends to be about 40% higher in cities where the average house price is a 1000 euro higher. *------------------ begin example ------------------ // get and prepare some data use "http://fmwww.bc.edu/repec/bocode/c/citybudget.dta";, clear gen leftright = minorityleft + 2*noleft label variable leftright /// "political orientation of members of city government" label define leftright 0 "majority left" /// 1 "majority right" /// 2 "only right" label value leftright leftright gen chouseval = houseval - 1.5 label variable chouseval /// "average value of a house in 100,000 euros (centered at 150.000 eureo)" gen cpopdens = popdens - .75 label variable cpopdens /// "population density in 1000s persons per km^2 (centered at 750 persons per km^2)" // estimate the model glm governing i.leftright chouseval cpopdens, /// family(binomial) link(logit) vce(robust) eform *------------------- end example ------------------- * (For more on examples I sent to the Statalist see: * http://www.maartenbuis.nl/example_faq ) Hope this helps, Maarten --------------------------------- Maarten L. Buis WZB Reichpietschufer 50 10785 Berlin Germany http://www.maartenbuis.nl --------------------------------- * * 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/

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