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varest<-function(Ys,Xs=NULL,pik,w=NULL)
{
if (any(is.na(pik)))
stop("there are missing values in pik")
if (any(is.na(Ys)))
stop("there are missing values in y")
if (length(Ys) != length(pik))
stop("y and pik have different sizes")
if(!is.null(Xs))
{if(is.data.frame(Xs)) Xs=as.matrix(Xs)
if(is.vector(Xs) & (length(Ys)!= length(Xs)))
stop("x and y have different sizes")
if(is.matrix(Xs) & (length(Ys) != nrow(Xs)))
stop("x and y have different sizes")
}
a=(1-pik)/sum(1-pik)
if(is.null(Xs))
{A=sum(a*Ys/pik)
var=sum((1-pik)*(Ys/pik-A)^2)/(1-sum(a^2))
}
else
{B=t(Xs*w)
beta=ginv(B%*%Xs)%*%B%*%Ys
e=Ys-Xs%*%beta
A=sum(a*e/pik)
var=sum((1-pik)*(e/pik-A)^2)/(1-sum(a^2))
}
var
}
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