I am using semiparametric Model
 library(mgcv)
sm1=gam(y~x1+s(x2),family=binomial, f)

How should I  find out standard error for ed50 for the above model 
ED50 =( -sm1$coef[1]-f(x2)) / sm1$coef [2]
 
f(x2) is estimated value for non parametric term.
 
Thanks


      
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