The basis between ABX subindex and wght average sum of underlying ref obligagtions

sometimes large because --investor base for single name ABCDS is dominated by long-only CDO protection sellers whereas index is more actively traded by corproate cross-over accounts and hedge funds --the bid offer in the underlying 20 tranches is large enough to prohibit potentail arbitrage between the index until the basis detween single names and index is very large.