3.3 | Random-Forest ABC scenario-choice for the history of ACB and ASW populations
We performed RF-ABC model-choice separately for the admixture history of the Barbadian (ACB) and the African American (ASW) populations, to evaluate whether our MetHis -ABC method could identify subtle differences in the history of both populations having experienced the TAST under the British colonial empire (Martin et al. 2017; Baharian et al. 2016). For the ACB, Figure 3 shows that the majority of votes (53.1%) went to an admixture scenario AfrDE-EurDE with a posterior probability of the winning scenario of 60.28%. This posterior probability is above the mean posterior-probability obtained when the wrong scenario is chosen for the 1000 AfrDE-EurDE simulations closest to the observed one (56.8%, SD=11.6%, for 37 simulations wrongly assigned in total). The second most chosen scenario was the AfrDE-Eur2P scenario. However, this scenario is voted for 3.5 times less often than the winning scenario AfrDE-EurDE, gathering 15.1% of the 1,000 votes, only slightly above the 11.11% prior probability for each nine-competing scenario (Figure 3 ; Supplementary Table S1 ).
RF-ABC scenario-choice results were less segregating for the ASW.Figure 3 shows that the AfrDE-EurDE scenario also gathered the majority of votes, albeit with lower posterior probability than for the ACB (33.5% of 1,000 votes, with posterior probability = 48.0%). This posterior-probability is slightly below the average posterior-probability obtained when the wrong scenario is chosen for the 1000 AfrDE-EurDE simulations closest to the ASW (50.7%, SD = 7.9%, for 192 simulations wrongly assigned). The second most chosen scenario, AfrDE-Eur2P, was only slightly less chosen with 31.7% of the votes (Figure 3 , Supplementary Table S1 ). Altogether these results denote an ambiguity of the RF-model choice in the part of the parameter-space occupied by the ASW.
Considering only these two best scenarios to train the RF and re-conducting scenario-choice improved the scenario discrimination in favor of the AfrDE-EurDE scenario. While we found, again, only a slight majority of votes (51.8%) in favor of the AfrDE-EurDE scenario, the posterior probability for this model was substantially increased to 57.9%, thus above the average posterior-probability threshold calculated above (50.7%). This indicated that the AfrDE-EurDE scenario best explained the ASW observed genetic patterns, despite overall limited discriminatory power of our approach in the ambiguous part of the summary-statistics space occupied by this population.