### Self-prophesying models

#### (Sep 2, 2008)

A phenomenon to watch for is that of the "self-prophesying model".  It occurs when a variable is too specific to the mortality experience of a reference portfolio to have wider application.  It has been claimed that the risk of this increases for smaller data sets and more lifestyle categories. In fact, the error is actually most likely where there is a small number of lives in each sub-group prior to grouping.  This is simply because the impact of random variation is largest in very small groups.

As an illustration of this, consider the apparently reasonable alternative of using postcode sector instead of a geodemographic type.  The postcode sector is basically everything except the last two characters…