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(Un)Fit for purpose
Demography's dark matter: measuring cohort effects
My last blog generated quite a bit of interest so I thought I'd write again on cohorts. It's easy to (a) demonstrate the existence of a cohort effect and to (b) fit models with cohort terms, but not so easy to (c) interpret or forecast the fitted cohort coefficients. In this blog I'll fit the following three models:
Forecasting with cohorts for a mature closed portfolio
At a previous seminar I discussed forecasting with the age-period-cohort (APC) model:
$$ \log \mu_{i,j} = \alpha_i + \kappa_j + \gamma_{j-i}$$
A second pension-scheme revolution
Spotting quality issues with limited data
Effective dimension
Actuaries often need to smooth mortality rates. Gompertz (1825) smoothed mortality rates by age and his famous law was a landmark in this area. Figure 1 shows the Gompertz model fitted to CMI assured lives data for ages 20–90 in the year 2002. The Gompertz Law usually breaks down below about age 40 and a more general smooth curve would be appropriate. However, a more general smooth curve would obviously require more parameters than the two for the simple Gompertz model.
Boundless confidence?
S2 mortality tables
The perils of parameter interpretation
With some notable exceptions, such as the Kaplan-Meier estimator, most mortality models contain parameters. In a statistical model these parameters need to be estimated, and it is a natural thing for people to want to place interpretations on those parameter estimates. However, this can be tricky, as parameters in a multi-parameter model are dependent on each other.