44.2 Frailty

In general, we define frailty as a multiplicative factor on the FoI. If the average FoI in the population is \(h\), then the FoI in a stratum is \(hk\). The size of the stratum, \(p_k\), is constrained such that:

\[\int_0^\infty k \; p_k \; dk = 1\]

With this constraint, the mean FoI in the population is \(h\).

Continuous distributions are difficult to extend, but we can stratify a population to accomplish some of the same effects.