# Observed information is used for Wald's test
# https://www.jstor.org/stable/pdf/2335893.pdf
# Bradley Efron and David V. Hinkley, Biometrika, 65:3, 457-482
EL.vcov <- function(par, dat){
J <- -deriv2(par, dat)/dat$n
rho0 <- dat$n0/dat$n
rho1 <- dat$n1/dat$n
map <- dat$map
vcov <- solve(J)
vcov[map$a, map$a] <- vcov[map$a, map$a] - (rho0 + rho1)/rho0/rho1
vcov <- vcov/dat$n
colnames(vcov) <- names(par)
rownames(vcov) <- names(par)
if(any(eigen(vcov)$values <= 1e-6)){
warning('EL may be misleading as vcov is not a positive-definite matrix')
}
vcov
}
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