There is no One True Way to do this, and different methods will have different limitations. An average of all the scores will include many samples that were deliberately chosen for health problems (The median IQ of all the samples in the table is 75). I’ve rated the samples for quality to help limit the influence of less representative test groups. 18 of the 41 samples were disadvantaged, 5 of the samples were advantaged, and 18 were reasonably representative. The median IQ of the disadvantaged samples was 66, while the median IQ of the advantaged samples was 87.
The median of 18 reasonably representative samples gives us an IQ of 80 for Jamaica. This is also consistent with Samms-Vaughan (2005 ), one of the more recent and methodologically robust studies, which shows an average IQ of about 81.
It would be interesting to see whether there's enough research on the small island of Barbados to come up with a reliable IQ comparison to Jamaica. There has always been a stereotype of Barbados as more educated and genteel than Jamaica, but does it show up in the data?