var.linear {boot}  R Documentation 
Estimates the variance of a statistic from its empirical influence values.
var.linear(L, strata=NULL)
L 
Vector of the empirical influence values of a statistic. These will usually
be calculated by a call to empinf .

strata 
A numeric vector or factor specifying which observations (and hence empirical influence values) come from which strata. 
The variance estimate calculated from L
.
Davison, A. C. and Hinkley, D. V. (1997) Bootstrap Methods and Their Application. Cambridge University Press.
empinf
, linear.approx
, k3.linear
# To estimate the variance of the ratio of means for the city data. ratio < function(d,w) sum(d$x * w)/sum(d$u * w) var.linear(empinf(data=city,statistic=ratio))