[R] Logistic regression for drugs Interactions

[francogrex]

I have the model below, for which I run a logistic regression including the
interaction term (NSAID*Diuretic)
------------------------
fit1=glm(resp ~ nsaid+diuretic+I(nsaid*diuretic), family= binomial,data=w)


NSAID	Diuretic	Present	Absent
0	0	185	6527
0	1	53	1444
1	0	42	1293
1	1	25	253




	Coefficients	Std. Error	z value	Pr(>|z|)    
(Intercept)	-3.56335	0.07456	-47.794	 < 2e-16 ***
NSAID	0.13630	0.17361	0.785	 0.43242    
Diuretic	0.25847	0.15849	1.631	 0.10293    
I(NSAID*Diuretic)	0.85407	0.30603	2.791	 0.00526 ** 
---
Signif. Codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 
Odds ratio is 2.35 [ln(0.85407)] times higher when NSAID is present in
addition to Diuretic.

-------------------------------

Odds ratio of Nausea when on Diuretic is exp(0.25847)= 1.29
and the odds ratio of Nausea when on NSAID is exp(0.13630)=1.14
Normally when we want to see the odds ratio of Nausea when a patient is on
both drugs we multiply 1.29*1.14= 1.48 (is this correct? do we multiply or
do we add?)

But since the interaction term is significant then we take that into
account? Does that mean that the odds ratio of the interaction is
exp(0.25847)*exp(0.13630)*exp(0.85407)=3.486297 ?

Or do we use additions?

Thanks.
-- 
View this message in context: http://www.nabble.com/Logistic-regression-for-drugs-Interactions-tf3404506.html#a9482343
Sent from the R help mailing list archive at Nabble.com.