How to play 'Make me a Market'

"Make me a market" pairs two skills in one exercise: estimate a quantity you have no data for, then quote a two-way price around your own estimate and defend it while someone trades against you. It turns up in final rounds at most market makers, and it is where candidates who have only ever practised silently tend to come apart.

This guide covers the structure of the game, how to read what the interviewer does, and how to keep track of your position and profit.

How the game works

1
The estimate. A Fermi-style question: estimate a quantity from limited information.
2
Your market. Quote a bid and an ask around that estimate, with the spread reflecting your confidence.
3
The interviewer acts. They buy at your ask, sell at your bid, or, in some variants, decline to trade.
4
You requote. Adjust your prices in light of what they just did.
5
Position and P&L. After a series of trades, state your net position and your total profit or loss.
6
The debrief. Explain your approach, your adjustments, and what you would do differently.

Step 1: The Fermi estimate

Fermi questions ask you to estimate a quantity with no direct data, using decomposition and defensible assumptions instead. They open this game because they test the thing market making needs: breaking a vague problem into parts you can actually reason about, and quantifying how unsure you are.

How to work one

Decompose first. Break the question into parts you can estimate. For the number of gas stations in a city, start from the population, then cars per household, then how many cars one station serves.
State your assumptions. Fill the gaps with reasonable, justifiable numbers, and say them out loud as assumptions rather than facts. "I will assume roughly half the population drives" is a strong move; a silent guess is not.
Answer in a range. A range communicates your uncertainty in a way a single number cannot. "Between 10,000 and 15,000" tells the interviewer how confident you are, which is exactly what your spread has to encode a minute later.

Practise the estimation half on its own before combining it with quoting. Our Fermi question trainer holds 157 of these, and the Fermi estimation game puts them under a clock. Piano keys, piano tuners and tennis balls are the classic three to start with.

Step 2: Making your market

Now you quote. Your bid is where you will buy, your ask is where you will sell, and the gap between them is your spread.

Setting the first quote

The spread is the decision that carries the most information about you. A wide spread protects you when your estimate is shaky, but quote it too wide and the interviewer has no reason to trade at all, which costs you the chance to show anything. A narrow spread signals confidence and punishes you hard if the estimate is off.

Key takeaway

Your spread should be an honest translation of the range you just gave. If you said "between 10,000 and 15,000" and then quote a spread two points wide, one of the two statements is not true, and that is the inconsistency the interviewer is looking for.

Reading what the interviewer does

Every action, including inaction, is feedback on your estimate.

They...	What it suggests	How to respond
Buy, taking your ask	They think the true value is above your ask, or they are testing how firmly you back your numbers	Reassess your assumptions for anything you left conservative, then move both quotes up gradually
Sell, taking your bid	They think the true value is below your bid	Re-check your estimate for overestimation, then move down, with reasoning you can state aloud
Do not trade	Usually implicit validation: they think the number sits inside your spread and find the range reasonable	Hold. If non-trades keep repeating, your spread is probably too wide to invite engagement, so tighten it slightly to test

Do not reprice off a single trade

One trade is one data point, and overreacting to it is the most common way candidates lose this game. Move gradually, keep the spread wide enough to protect the position you now hold, and be ready to justify every adjustment. Adjustments backed by reasoning preserve your credibility; adjustments that look like flinching do not.

When the interviewer never declines

Some interviewers drop the non-trade option entirely and simply keep trading as long as your range contains the real number, to see how you handle continuous flow. If you spot this, exploit it: quote around your estimate so they buy and sell in roughly equal amounts, and you collect the spread on every round trip.

When the interviewer is making mistakes

Some will deliberately trade badly to see whether you notice and act. If you find a way to profit consistently from irrational behaviour, take it immediately rather than politely ignoring it. The inverse also happens: some interviewers will quietly profit from your mistakes and then ask you about it afterwards, so keep track.

Step 3: Position and profit

Knowing where you stand at every moment is most of what is being assessed here.

Keep a running record

Log every trade: quantity, price, and whether it was a buy or a sell. A simple mental ledger with buys on one side and sells on the other is enough, and you cannot reconstruct it later if you skip it.
Track your net position continuously. Net long if you have bought more than you have sold, net short the other way. Net long means you profit if the true value comes in above your estimate; net short means you profit if it comes in below.

Calculating your profit

Market making earns the spread: buying at your bid and selling at your ask. Each round trip is the quantity times the difference between the two prices, summed over every trade.

Total profit

$\text{Profit} = \sum_i q_i \left( p^{\text{sell}}_i - p^{\text{buy}}_i \right)$

Then mark any position you are still holding to the final value.

If the game ends with you net long or net short, expect to be asked for the theoretical profit or loss on that residual position given the final estimated value. Being able to close that loop is the difference between having played the game and having understood it.

The debrief

The questions afterwards are part of the assessment, not a formality.

Walk back your trades. Were the assumptions sound? Was each response to the interviewer proportionate?
Say how you read the signals. Where did you interpret a trade correctly, and where did you over-read one?
Assess your spread management. Too wide and you discouraged trading; too narrow and you carried risk you had not priced. Saying which one you erred toward is a stronger answer than claiming you got it right.

How to prepare

The two halves of this game are best drilled separately, then together.

Estimation: the Fermi trainer and the Fermi estimation game.
Quoting and position management: the Market Making Dice Game, where you set spreads and carry inventory, and the Trading Game for quoting under a moving market. The Card Game drills the other side, taking a price when you see edge, which sharpens your read on why an interviewer trades against you.
The reasoning underneath it: our game theory and strategic trading course, which covers why the counterparty's decision to trade is itself information, how far a single fill should move your price, and how to size what you show.
Say it out loud. This game is played verbally, and quoting in your head builds none of the fluency you need. Narrate the fair value, then the quote around it, every time you practise.
Get feedback. Mock it with someone who will actually trade against you and question the adjustments afterwards.

Firms that lean on this exercise are worth reading up on individually: SIG builds its final stage around expected value games played in the room, and our full interview guide covers where the exercise sits in the wider process. All our market games are in one place.

Closing thoughts

Many firms play this slightly differently, so treat the structure above as the shape rather than the script. What does not vary is what is being measured: whether you can build an estimate you can defend, price around it honestly, update without panicking, and tell someone afterwards exactly what you did and why.

Success here is not finding the right answer. It is showing a structured, logical, adaptable approach to a problem nobody has the answer to.