An exploration of the correlation among the three investigated phenotypes could be helpful to interpret the consistency (or lack of consistency) among the association results. The correlation between CPD and ND was low for both AAs (Spearman correlation coefficient ρ = 0.14) and EAs (ρ = 0.19). A low correlation between CPD and FTND was also observed in AAs (ρ = 0.16) and EAs (ρ = 0.26). The low correlation between CPD and ND/FTND could explain the inconsistent results for rs2236196, which demonstrated association for CPD but not for ND or FTND. There is a strong correlation between the ND and FTND phenotype (ρ = 0.83 and 0.88 in AAs and EAs, respectively), which could explain the similar patter of association for ND and FTND (Table 3).
