The most popular measure of richness which takes a form (with weights \(w_1,w_2,...,w_n\)):
$$R^{IS}(\boldsymbol{x},\boldsymbol{w},p) = \frac{\sum_{i=1}^n{x_iw_i}\boldsymbol{1}_{x_i>q_{w(1-p)}}}{\sum_{i=1}^n{x_iw_i}},$$
where \(q_{w(1-p)}\) is the \((1-p)\) quantile of the population and \(\boldsymbol{1}_{(\cdot)}\)
denotes the indicator function, which is equal to 1 when its argument is true and 0 otherwise.
There is always \(p\) % of rich individualsa in the population.