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st: Re: attributable fraction


From   Marc Campo <mcam54@yahoo.com>
To   Marc Campo <mcam54@yahoo.com>, statalist@hsphsun2.harvard.edu
Subject   st: Re: attributable fraction
Date   Fri, 3 Nov 2006 12:32:19 -0800 (PST)

I have calculated several attributable fractions. 
When the AF comes up as negative however, Stata does
not display a standard error and confidence interval. 
Why?

. logistic incident_lowback age gender hours_week
second_job strain xfer posi manual

						
incident_l~k	Odds Ratio	Std. Err.	z	P>z	[95% Conf.
Interval]
						
age	.9849692	.0149313	-1.00	0.318	.9561349	1.014673
gender	.9118571	.3196976	-0.26	0.792	.4586644	1.812836
hours_week	1.000273	.0169735	0.02	0.987	.9675525
1.034099
second_job	1.0135	.3644665	0.04	0.970	.5008663
2.050812
strain	2.311378	.7130899	2.72	0.007	1.262595	4.231337
xfer	1.658223	.4354864	1.93	0.054	.9910564	2.774517
posi	.7943614	.2137511	-0.86	0.392	.4687834	1.346059
manual	1.025606	.0583063	0.44	0.657	.9174643	1.146494


. aflogit posi, ref(posi=1)

Term	Ref.	A.F.	s.e.	[95%	Conf.	Int.]*
						
posi	1	-0.2742	.		.	.
						
TOTAL		-0.2742	.		.	.


Any thoughts?  Why can't you have a negative AF?

Marc
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