IN(d,s)=I(d,s)I(d)=I(d)−I(d∣s)I(d)=1−∑si∈S∑x∈Vdp(si,x)logp(x∣si)∑x∈Vdp(x)logp(x)
where p(x) is the probability of observing x in U, p(si,x) is the probability of observing si and x in U, and p(x∣si) is the probability of observing x given that the stratum is si.