var_pois estimates the variance of the estimator
of a total for a Poisson sampling design.
var_pois(y, pik, w = NULL)A numerical matrix of the variable(s) whose variance of their total is to be estimated. May be a Matrix::TsparseMatrix.
A numerical vector of first-order inclusion probabilities.
An optional numerical vector of row weights (see Details).
The estimated variances as a numerical vector of size the number of columns of y.
w is a row weight used at the final summation step. It is useful
when var_pois is used on the second stage of a two-stage sampling
design applying the Rao (1975) formula.
Rao, J.N.K (1975), "Unbiased variance estimation for multistage designs", Sankhya, C n<U+00B0>37