Dark-pool routing and adverse selection
Your desk must buy 2.5 million shares of a $4.8 billion mid-cap industrial over three days. Average daily volume is 4 million shares and the spread is 3 cents. The smart order router sends 55% of child orders to a dark venue that reports a 38% fill rate, well above the 22% peer average. Post-trade analysis shows the stock rises 0.18% in the 30 minutes after dark fills and falls 0.04% after lit fills. The router's routing table was last reviewed 14 months ago, and a new venue paying a $0.0025 per-share rebate now receives 10% of flow. The portfolio manager wants to finish in two days to capture a momentum signal. Implementation shortfall is 24 basis points versus an 11 basis point benchmark, and venue-level reports are produced monthly. Compliance asks whether the desk can justify the dark routing under best execution.
1. What is the most likely explanation for the high dark fill rate and the post-fill price moves? Is the 38% fill rate evidence of good execution?
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High fill rate is not automatically good. The asymmetry, +18 basis points after dark fills and -4 basis points after lit fills, is a classic adverse-selection signature: the dark venue is disproportionately filling the desk when the counterparty is informed or when the price is about to move against the buy order. The 22 basis point spread between the two markouts is economically large relative to the 3-cent spread and the 11 basis point benchmark. A 38% fill rate can mean the venue is generously filling toxic orders, not that it is finding liquidity at good prices.
Other explanations to test: the venue may have a different order-type mix; the desk may be sending more marketable child orders there; or the monthly reports may lag. The 55% dark allocation, a routing table last reviewed 14 months ago, and a new rebate venue receiving 10% of flow also raise drift and conflict concerns. Conclusion: investigate before defending. The current routing is likely inconsistent with best execution.
2. What execution and routing changes would you make, and how would you handle the PM's two-day deadline?
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Separate the liquidity need from the alpha urgency. For the next phase, cut dark participation sharply, especially for larger child orders; route more to lit venues; use limit orders with a cap; target participation around 10-15% of volume; and use the closing auction for residual volume. Exclude or quarantine the venue with the worst markouts until a formal review. Do not let the rebate venue receive flow without a conflict-of-interest and fill-quality assessment.
For the PM, explain that compressing to two days may require raising participation to 20-25% of average daily volume, which increases impact and signals intent. Offer a two-day schedule only if the PM accepts a limit price and the desk can use conditional or pegged orders. If the momentum signal is strong, consider a risk transfer such as a block or a derivative rather than uncontrolled market orders. Document the trade-off: implementation shortfall is already 24 basis points against an 11 basis point benchmark, and speeding up without changing routing may worsen it. Best execution is not the same as fastest completion.
3. What evidence should the desk present to compliance to demonstrate best execution, and what ongoing monitoring is needed?
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Present venue-level and order-level evidence: effective spread, realized spread, 5-, 30-, and 60-minute markouts, fill rate, reject rate, latency, order-type mix, and price improvement, segmented by order size and urgency. Show peer and venue comparisons and the rationale for the dark allocation. Demonstrate that the routing table is reviewed at least quarterly and that rebates are disclosed and controlled.
Ongoing monitoring should include daily exception reports on markouts by venue, monthly best-execution committee review, independent validation of the smart order router logic, and change control for new venues. Document client instructions and any deviation from default routing. Compliance should also test whether routing is influenced by rebates rather than execution quality. If the 18 basis point adverse markout persists, remove the venue and consider a remediation note to the best-execution committee.