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From |
"David Winter" <d.l.winter@bham.ac.uk> |

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

Subject |
st: stratifying after regression |

Date |
Wed, 9 Nov 2005 13:25:27 -0000 |

My colleague is having problems subscribing to Statalist. He asked me to post this on his behalf. Dave *********************************************************** Mr David L. Winter, HNC IT Manager (BCCSS Study) Department of Public Health & Epidemiology University of Birmingham Edgbaston Birmingham B15 2TT UK tel: +44 (0)121 4146766 fax: +44 (0)121 4147923 *********************************************************** -----Original Message----- From: Raoul C Reulen Sent: 09 November 2005 11:44 To: David Winter Subject: FW: stratifying after regression Hello, Let's say you want to compare the blood pressure between two groups by using multivariable linear regression, and the model that looks like this; .xi: reg BP i.age i.sexe group i.age*group Where: "BP" is blood pressure "Group" is coded as 0 (group A) and 1 (group B) Now, because of the interaction between age and group, I want to report the difference in mean blood pressure between the two groups stratified by age, but still adjusted for sex. What is the way to do it? A) by creating several regression models for every age group, where the regression coefficient of the variable "group" is the mean difference in blood pressure between the two groups: .xi: reg BP i.sexe group if age==1 .xi: reg BP i.sexe group if age==2 .xi: reg BP i.sexe group if age==3 etc. or B) by using the full model .xi: reg BP i.age i.sexe group i.age*group and using .adjust to predict the means of group for every age group: .adjust sex if age==1 , by(group) .adjust sex if age==2 , by(group) .adjust sex if age==3 , by(group) etc. The results differ slightly because in example A you use data for the variable "sex" only for the age group of interest, but in example B you use all the data on "sex" (so for all age groups). What is the correct way? Thanks for the help. Raoul * * 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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