Market structure and bid-ask spreads: IBIS vs Nasdaq

A comparison is made between the bid-ask spreads of 30 high volume German stocks traded on IBIS and 30 high volume US stocks traded on Nasdaq. IBIS and Nasdaq are best described as agency and dealer auction markets, respectively. On average, the market spread for these IBIS and Nasdaq stocks is the same, but for the 10 most active stocks in each market, IBIS spreads are considerably lower. For these latter stocks, IBIS spreads change in a predictable manner throughout the day. Nasdaq spreads do not. The critical factor appears to be the unrestricted access of suppliers of immediacy that is distinctive for agency auction markets.