LAGOS — Thirteen large companies on the Nigerian Exchange lost N3.7 trillion in market value in one week as the All-Share Index fell 1.2 per cent. Analysts linked the decline to the Central Bank's decision to open OMO bill auctions to individuals, at rates of up to 20.4 per cent, and to a change in the exchange's pricing method. BUA Foods fell 11.1 per cent and MTN Nigeria 4.97 per cent.