Actuarial news and views from Cape Town and beyond

The ASSA2008 Model


The ASSA2008 model was released in March of 2011, and is the most recent version of the ASSA AIDS and Demographic model to be released. The most significant changes to the previous version of the model are greater estimates of the impact of antiretroviral treatment in recent years, as well as significant increases in levels in condom usage over the last decade.

There was a link to the model on the Actuarial Society’s website, however it has been taken down. There are instructions on how to obtain the model here:

I thought it would be interesting to point out that there are a number of shortcomings in the ASSA2008 model. These include:

1. many of the empirical assumptions about sexual behaviour have limited empirical basis
2. HIV transmission probabilities are calculated on an annual basis, making it impractical to allow for transmission dynamics that operate over short periods due to the high level of infectiousness during the first few months of HIV infection.
3. problems with calibration to some of the age-specific calibration targets, for example the model treats the life expectancy quite conservatively
4. the model does not allow for the effect of recent changes in guidelines for ART initiation in adults and children, and the effect these have on MTCT (mother to child transmission) rates
5. the model does not allow for new PMTCT (prevention of mother to child transmission) guidelines, particularly provision of extended NVP (nevirapine) prophylaxis to children breastfed by HIV+ mothers
6. the model does not allow for the effect of the campaign to promote male circumcision
7. the model does not allow for the effect of recent HIV testing campaign (12 million tested in 2010/11)
8. the model does not allow for new strategies that may be introduced in the future (microbicides, PrEP, earlier ART initiation)
A new ASSA model is being developed using the THEMBISA model and will address many of the shortcomings of the ASSA2008 model.


4 thoughts on “The ASSA2008 Model

  1. Very relevant post and apt comments, thanks!!

  2. So I don’t know if you all know but Rob Dorrington here at UCT is one of the creators of this model!

    So what you might want to think about is:
    – What does the output of this model look like? What do you think an AIDS model produces?
    – How is this model used to price insurance products, if at all?
    – How do you think the various shortcomings affect the results of the model?

  3. Hi colleagues, the model is in fact a big workbook that is built on MS Excel. It actually produces a number of mid-year demographic estimates, and I might not be able to name them all on this platform. But the ones I think are of more interest to actuaries include the estimates of yearly HIV incidence and prevalence rates, population numbers by sex and age, mortality estimates, among others. The ‘Results’ sheet of the model carries all these numbers with years as columns and rows as names assigned to estimates. The model is in fact a suite of models since it has multiple versions, namely the national ‘full’ model and national ‘lite’ model, together with the provincial versions too.

    The model is used by actuaries to provide bases for pricing and valuation, and the aim of it is to allow actuaries to factor in AIDS in their derivation of actuarial bases in the South African life offices and pensions funds, but you see, that’s as far as I can understand how actuaries use the model. Actuaries themselves can tell us a lot more about how they use it.

    This model also provides life tables (mortality tables) for anyone who may wish to make use of those.

    The various shortcomings of the model, as raised by Michael, first of all, affect the results of the model in the sense that the calculus of the epidemic is treated conservatively in the model, hence the mid-year estimates of life expectancy at birth, and the subsequent projection thereof, are low, the mortality estimates are heavier than they should be, I suspect, and fertility estimates are lighter than they should be.

    Finally, the model works like a net migration model when you consider provincial versions. This is just one methodology option in the arsenal of the demographer seeking to perform population projections. The model can be adapted in its VBA code and re-configured to work like a multi-regional population projection tool to allow for a more transparent treatment of inter-provincial migration.

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