Oct 08, 2026 by Offer

IMC Order Book Question 2027: How to Analyze Trades

The IMC Trading order book question — commonly reported by candidates as "Here is a list of trades from an order book. How could we trade this better?" — is answered with an analysis framework, not instant brilliance: measure what happened (effective spread, slippage, timing), diagnose the costs, then propose specific improvements.

What This IMC Trading Order Book Question Assesses

Execution analysis is core market-making work — IMC wants to see whether you think about trading costs rigorously. Candidates who jump to "use smarter algorithms" without first measuring anything show buzzword instincts; candidates who start with "let's quantify what we paid and why" show quant instincts.

How to Answer This IMC Trading Order Book Question

Work the framework in order:

Step 1 — Measure. Compute the effective spread paid: how far from mid were the fills? Measure slippage against arrival price and break it down by time of day, trade size, and venue. Example line: "First I'd quantify the damage — effective spread per trade, slippage versus arrival mid, sliced by size and time."
Step 2 — Diagnose. Where is the cost concentrated? Large trades moving the market (impact), trading at volatile times (timing), crossing wide spreads unnecessarily (urgency)? The pattern in the costs points at the fix.
Step 3 — Propose. Match fixes to diagnoses: slice large orders over time if impact dominates, avoid volatile periods if timing dominates, use passive limit orders where the spread cost dominates. Each proposal tied to a measured cost — never generic.
Step 4 — Validate. Propose how you'd test it: paper-trade the new logic on historical data, A/B against the old approach. "And I'd measure whether it actually helped" is the sentence that shows professional discipline.

The through-line: measure first, diagnose second, prescribe third. That order is the entire answer.

Common Mistakes

Prescribing before measuring. Recommending TWAP or smarter routing without knowing what the costs were is consulting-speak, not quant work.
Ignoring the benchmark. "Better" needs a baseline — slippage versus arrival price, or versus a scheduled benchmark. Without it, improvement is unmeasurable.
One-size-fits-all fixes. The right execution depends on the diagnosed cost; a single favorite algorithm applied blindly is a red flag.

Candidates commonly report follow-ups like "What if the slippage is all in the first five minutes?" — that's a timing diagnosis demanding a timing fix (delay or slice the open). The follow-ups test whether your framework flexes with the data, so keep the measure-diagnose-prescribe order explicit.

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FAQ

What is effective spread? Twice the distance between your fill price and the mid-price at the time — the true cost of demanding liquidity, as opposed to the quoted spread.

What's the difference between slippage and market impact? Slippage is total deviation from your benchmark price; market impact is the portion your own trading caused. Decomposing the two tells you what you can fix.

Should I mention specific algorithms? Only after the diagnosis — TWAP for impact, passive posting for spread costs, opportunistic for quiet periods. Algorithms are treatments; name the disease first.

How would I present this in the interview? Think aloud through the framework, asking for the data you'd want at each step. The interviewer is grading the structure of your thinking, not a final answer.

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