Bid-Ask Spread

The foundational layer of the spread covering the direct operational expenses incurred by liquidity providers. This includes exchange access fees, clearing and settlement costs, and technology infrastructure overhead. In the maker-taker fee model, these costs are partially offset by rebates. For a retail market maker, this component typically accounts for 10-25% of the total spread, varying with trade size and venue complexity. Unlike other components, order processing costs are relatively fixed and predictable, forming the non-negotiable floor of the spread.

Compensation for the risk that a market maker's inventory position depreciates before it can be unwound. When a market maker buys on the bid, they assume directional price risk until they can sell. The more volatile the asset and the longer the expected holding period, the wider this component becomes. Key factors include:

• Volatility: Higher realized volatility demands wider spreads
• Correlation: Portfolio effects with existing inventory
• Holding time: Expected duration until offsetting trade This component is dynamic, widening sharply during news events and volatility spikes.

The premium charged to protect against trading with informed counterparties who possess superior information about the asset's true value. This is the most theoretically significant component. When a market maker quotes a price, they risk being 'picked off' by traders who know the price is stale. The VPIN metric and realized spread are used to measure this cost empirically. Markets with high levels of informed trading exhibit wider spreads. This component explains why spreads widen before earnings announcements and narrow when information asymmetry is low.

The excess spread attributable to imperfect competition among liquidity providers. In markets with monopolistic or oligopolistic market making, spreads exceed the sum of costs and risks. This component shrinks when:

• Multiple market makers compete aggressively
• Electronic trading reduces barriers to entry
• Regulatory pressure enforces best execution In highly competitive electronic markets like large-cap equities, this component approaches zero. In fragmented or opaque markets, it can be substantial.

The regulatory or exchange-mandated minimum price increment that artificially floors the spread. When the economic spread would naturally be narrower than the tick size, the constraint becomes binding. For example, if the true economic spread is 0.1 cents but the tick size is 1 cent, the quoted spread is forced to 1 cent. The SEC's Tick Size Pilot Program studied this effect extensively. Reducing tick sizes generally narrows spreads but can reduce displayed liquidity depth, creating a complex trade-off between spread width and depth.

For fixed-income securities, the spread over the risk-free curve after stripping out the value of embedded options. This isolates the pure credit and liquidity premium. The calculation involves:

• Monte Carlo simulation of interest rate paths
• Option-adjusted duration and convexity
• Prepayment modeling for mortgage-backed securities OAS allows apples-to-apples comparison of bonds with different call, put, or prepayment features. A widening OAS signals deteriorating credit quality or liquidity, while a narrowing OAS indicates improving conditions.