The function `var_f` computes the variance.
var_f(ascale_r, ascale_nr, tau, bshape, ascale_cens, p)scale parameter for the Weibull distribution for responders
scale parameter for the Weibull distribution for non-responders
follow-up
shape parameter for the Weibull distribution
distributional parameter for the exponential distribution for the censoring
event rate for the response
Variance computation