An E-mail from Rob Low

A brief comment on Ferguson's formula. It is obviously open to fairly silly misuse. For example, you quote a negative correlation between Mobius Test scores and Mega scores: it would be easy to combine test scores on the two and get a ridiculously inflated result simply by failing to take into account the fact that the mean of the samples was up in the region of IQ 150, and that the computation had to be done taking this into account.

That said, it shouldn't be difficult to give an explicit means of combining all the acceptable tests--with only three tests, there are only four combinations to worry about, and the data must exist for doing something moderately sensible regarding the correlations.