GSTM1 and malaria: Performance of different imputation models in terms of genetic effect estimation: IM0 refers to imputation carried out using the observed frequencies of the α3.7-globin deletions, IM1 includes four SNPs as imputation covariates (rs1800629, rs3211938, rs334, and rs542998), IM2 includes eight phenotypes and socio-environmental factors (Hb, mild anemia, malaria parasite positivity, transect, altitude, and ethnicity), and IM3 includes all variables in IM1 and IM2