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

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Regression Across Two Groups |

Date |
Wed, 14 Dec 2011 11:12:02 +0100 |

On Wed, Dec 14, 2011 at 10:36 AM, Muhammad Anees wrote: > Yes, the earnings (the dependent variable) is a group variable where > groups represent different levels of earnings (0-5000, 5001-10000, and > so on). I can treat such types of variables with confidence using GLM > type regression, but I was concerned with what techniques are > available to compare different regressions. As I said before, just add the appropriate dummies and/or interactions. Consider the example below. In this case we want to compare foreign and domestic cars.The expected price of a domestic car with median mileage and repair status is 5,419 dollars. This price increases by (1.16 - 1)/100%=16% if the car is foreign. This is the difference(*) in constants of a regression on only foreign cars and a regression on only domestic cars. For domestic cars a unit increase in repair status leads to a non-significant (.96-1)*100%= -4% decrease in price, while a unit increase in mileage leads to a significant (.92-1)*100%= -8% decrease in price. The effect of repair status increases (becomes more negative) by a non-significant (1.16-1)*100%= 16% if the car is foreign, and the effect of mileage increases (becomes more negative) by a just significant (1.05-1)*100%= 5% if the car is foreign. The latter two effects reflect the difference(*) between regression coefficients if you estimated separate model for foreign and domestic cars. In this example I chose two continuous variables (well I treated rep78 as continuous, which is doubtful, but I did not want to interpret too many parameters). However, this works in the same way if you have categorical or ordered independent variables. Just precede those variable with -i.- instead of -c.-. (*) Notice that "difference" refers here to a quantification of "unequalness" rather than a difference in the mathematical sense, i.e. it is not one parameter minus the other parameter but rather the ratio of the two parameters. This is a general property of models using a log link, they compare groups via ratios rather than differences. *------------ begin example ---------------- sysuse auto, clear // median mpg = 20 gen cmpg = mpg - 20 // median rep78 = 3 gen crep78 = rep78 - 3 glm price i.foreign##(c.crep78 c.cmpg) , /// link(log) 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 Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Regression Across Two Groups***From:*Muhammad Anees <anees@aneconomist.com>

**RE: st: Regression Across Two Groups***From:*Cameron McIntosh <cnm100@hotmail.com>

**Re: st: Regression Across Two Groups***From:*Muhammad Anees <anees@aneconomist.com>

**Re: st: Regression Across Two Groups***From:*Richard Williams <richardwilliams.ndu@gmail.com>

**Re: st: Regression Across Two Groups***From:*Muhammad Anees <anees@aneconomist.com>

**Re: st: Regression Across Two Groups***From:*Maarten Buis <maartenlbuis@gmail.com>

**Re: st: Regression Across Two Groups***From:*Muhammad Anees <anees@aneconomist.com>

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