The Effect of Decimalization on the Components of the Bid-Ask Spread

Abstract

Previous empirical studies that decompose the bid-ask spread were done when securities traded in discrete price points equal to one-sixteenth or one-eighth of a dollar. These studies concluded that inventory and adverse-selection costs were economically insignificant compared to order-processing costs. Natural questions arise as to: (i) whether price discreteness allowed market makers to enjoy excess rents, thus reducing the significance of the inventory and adverse selection costs; (ii) whether discreteness decreased the traders' incentives to gather information; or (iii) whether methodologies previously employed mis-estimated the inventory and the adverse-selection costs. We show that the recent conversion to decimal pricing results in significantly tighter spreads. However, the dollar value of spreads attributed to adverse selection and inventory costs do not change significantly. Almost all of the reduction occurs in the order-processing component. As a result, inventory and adverse-selection costs now account for a significantly larger proportion of the traded spreads. A plausible explanation is that the minimum tick size constraint previously in place under fractional pricing allowed market makers to enjoy spreads that were larger than their actual costs.