Estimation of mortality rate at time $t$, GG91

page 41, Sacher 1956, 1966

  \mu(t) = 1/(2 dt) log( \frac{s(t-dt}{t+dt} )

Other methods discussed are: Cutler and Ederer 1958

GG91 argued that Saccher56 formula is better than CutlerEdere58 method.