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The reason some presidents are listed twice and some not listed at all has to do with the statistical technique (cluster analysis) used to do the groupings. Cluster analysis puts things together in groups. The groups are not defined in advance, the fall out from the commonalites of the data. So a bunch of points close together will from a group. If there is one president that has characteristics between two groups, such that he looks like he is kind of in group A and kind of in group B, he will be listed in both. Presidents who's characteristics are unique will fall away from any group and won't be listed.
I think they purposefully left GW off the list since he is still in office.
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