Not speaking for Uncertain, but I think the point here is that it’s possible to guess a person’s present age by picking up cues from the apparent age of the photograph and/or the apparent age of the person in the photograph (and thus their chances of being alive). A best-case scenario would be to have a suite of photographs all taken at the same time, whose subjects are the same age–a 1950 yearbook, for example, would be ideal.
Failing that, there should be some other way of systematically eliminating any bias due to apparent ages. One way would be to pair photos, so that each photo in a pair is taken at roughly the same time, and of people roughly the same age, and one photo in each pair is “alive” and one is “dead.”
It shouldn’t matter, but I think the method of picking the mix should be explicitly stated up front. Either have a set number of people in each catagory, or randomly pick the number in a pre-arranged way.
Something like yearbook would be the best, but that relies on some individual to do some substantial research.
As an alternative, we could use paired pictures. If enough people have sets of parents or grandparents where one is alive and one is dead, that would make a nice pair. Except for the fact that women tend to live longer than men…