Hi

I have fit a model, and estimated the risk difference(RD) for the
interaction term,but I don't know how to estimate the standard error and
confidence interval for RD. The code are as follows
 table(dat$A3)
#     0      1
#167762   4444
table(dat$A17)
#     0      1      2      3
#129982  23429  15880   2915
table(dat$A10)
#     0      1
# 39871 132335
table(dat$A10,dat$A17)
#         0      1      2      3
#  0  29833   5626   3685    727
#  1 100149  17803  12195   2188
fm.add = glm(A3~ A10 + A17 +A10*A17,data=dat, family=binomial(link =
"identity"))
coefs.fm.add = summary(fm.add)$ coefficients
coefs.fm.add
                 Estimate  Std. Error     z value      Pr(>|z|)
(Intercept)  0.0343244059 0.001054068  32.5637454 1.337835e-232
A101        -0.0125967800 0.001150347 -10.9504155  6.614351e-28
A171         0.0070904537 0.002857888   2.4810117  1.310101e-02
A172        -0.0039309187 0.003017991  -1.3024951  1.927472e-01
A173        -0.0123161528 0.005542338  -2.2221944  2.627017e-02
A101:A171    0.0006713323 0.003160276   0.2124284  8.317728e-01
A101:A172    0.0066395373 0.003357877   1.9773022  4.800748e-02
A101:A173    0.0180108304 0.006566514   2.7428297  6.091227e-03

#estimate 'Risk difference of A10 when A10 = 1,A17=1'
rd.1.1= coefs.fm.add[2,1] + coefs.fm.add[3,1] + coefs.fm.add[6,1];
#estimate 'Risk difference of A10 when A10 = 1,A17=2'
rd.1.2= coefs.fm.add[2,1] + coefs.fm.add[4,1] + coefs.fm.add[7,1];
#estimate 'Risk difference of A10 when A10 = 1,A17=3'
rd.1.3= coefs.fm.add[2,1] + coefs.fm.add[5,1] + coefs.fm.add[8,1];
> rd.1.1
[1] -0.004834994
> rd.1.2
[1] -0.009888161
> rd.1.3
[1] -0.006902102

Who can help me? Thanks.


-- 
Zongfu Cao
Peking Union Medical College

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