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From |
"Weichle, Thomas" <Thomas.Weichle@va.gov> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
Re: st: Interpreting coefficients for a gamma regression with log link (Stata 11) |

Date |
Mon, 19 Sep 2011 10:17:53 -0500 |

Hi Hitesh, As you know may know, the output being displayed is different depending on whether you specify the eform option or not. The resulting model is the same but the interpretation of the coefficients is slightly different. Suppose x is a 1/0 variable. Then, b = ln(E(y|x=1))/ln(E(y|x=0)) is the unexponentiated coefficient for x and e(b) = e^[ln(E(y|x=1))/ln(E(y|x=0))] = e^[ln(E(y|x=1))]/[e^ln(E(y|x=0))] = E(y|x=1)/E(y|x=0) is the exponentiated coefficient for x. When dealing with cost as an outcome, Blough (2000) states that the ratio directly interpreted from the exposure variable may be preferable to point estimates (unexponentiated coefficient). Returning to your specific questions: 1) You are correct in stating that the exponentiated coefficient of 1.031383 represents the arithmetic mean ratio in the dependent variable between HIV+ and HIV- patients [specifically mean(hiv=1)/mean(hiv=0)]. 2) Since 'insurance_3' is the reference category, the exponentiated coefficient for 'insurance_1' is interpreted as the following ratio: [mean(insurance_1)/mean(insurance_3)]. Tom Weichle Math Statistician Center for Management of Complex Chronic Care (CMC3) Hines VA Hospital, Bldg 1, C202 708-202-8387 ext. 24261 Thomas.Weichle@va.gov * * 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/

**Follow-Ups**:**Re: st: Interpreting coefficients for a gamma regression with log link (Stata 11)***From:*Hitesh Chandwani <hchandwani.stata@gmail.com>

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