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From |
Torben Højmark Sørensen <TorbenHojmarkSorensen@dsi.dk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Group mean comparison with CI's from empty fe model |

Date |
Tue, 14 Aug 2007 16:28:03 +0200 |

Hi, I'm a health economist and I'm trying to compare mean values af length of stay for a specific patient group between 8 hospital departments by calculating CI's. I'm not an econometrics expert and I hope someone can help me with understanding a suggested (from a friend) program-code for creating CI's of the fixed effect department means (deviation from total mean): The suggested fixed effect regression equation without any regressors: yij = c + uj + eij, where c is the total mean of length of stay for all patients and uj is the deviation from c for department j=1-8. ******************************************************* xtreg length_of_stay, i(hosp) fe predict hosp_res, u bysort hosp:tabstat hosp_res collapse (mean) hosp_res, by(hosp) gen sigma_e=2.6950446 // from the fe results * patients pr. hospital department gen pt=50 replace pt=23 if hosp==1501 replace pt=27 if hosp==3000 replace pt=45 if hosp==4202 replace pt=47 if hosp==6007 replace pt=34 if hosp==7026 replace pt=25 if hosp==7601 replace pt=22 if hosp==8001 gen se_hosp_res=(sigma_e/sqrt(pt)) gen L = hosp_res - 1.96*se_hosp_res gen H = hosp_res + 1.96*se_hosp_res ******************************************************** My question is, that I don't understand the SD for the department specific means "sigma_e/sqrt(pt)"? Is it calculated? Why is sigma_e used and not the traditional formula for SD=sqrt(E(uij-uj)^2)? Cheers, Torben * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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