Mean on subsets of a subset
DT[, mean(V2[V2 < quantile(V2, 0.25)]), by = V1] V1 V11: AR 0.0252: AD 0.1303: BD 0.0204: BX NaN5: CX NaN6: DX 0.2207: DD 0.0908: DR 0.020DT[, mean(V2[V2 > quantile(V2, 0.75)]), by = V1] V1 V11: AR 0.332: AD 0.283: BD NaN4: BX NaN5: CX NaN6: DX 0.447: DD 0.178: DR 0.28