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st: difference in lfit slop


From   Lars Folkestad <lfolkestad@health.sdu.dk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: difference in lfit slop
Date   Tue, 4 Oct 2011 15:30:16 +0200

dear list
i am doing a twoway scatte:

twoway (scatter methode1 age) (scatter metode2 age) (lfit methode1 age) (lfit methode2 age)

this gives two different lfit slopes, one for metode1 and one for metode2 by age. i would like investigate if this slope is different. i understand that i could investigate the coef from the two liniar regressions:

regress metode1 age
regress metode2 age

using the methode described by clogg et al:

 z= (b1-b2)/squ(SEb1e2-SEb2e2)

where b1 is the estimated slope1 (metode1) and SEb1 is the standard error for metode1.

my questions:
how do i do this in stata?
is the slope1 (b1) the same as _cons dfrom the metode1 regression model
in the litterature, i've seen that z-values above 2 in large samples are signifikant. my sample is petit.
so what z value is enough to discard rhe null hypothisis= there is no difference between the groups.

thank you for your answer

Lars Folkestad


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