Expected value, memory and honest self-assessment in one exercise — including the uncomfortable part where you have to report your own losses correctly.
Start the Cards Market Making drillFour other drills are free to try firstThree cards are dealt face-down and a market is quoted on their sum. Fair value is 3 × the average value of the cards still in the deck.
With a full deck and ace high, card values run 2 to 14, averaging exactly 8 — so fair value is 24. With ace low they run 1 to 13, averaging 7, giving 21.
If the deck depletes between rounds, that average drifts, and the quote does not always follow. Tracking the drift is where the edge lives.
After trading, the cards flash briefly and flip back. You then type what you made or lost. Correct profits are credited. Misreported profits are forfeited. Misreported losses cost double.
That asymmetry is deliberate and it mirrors real desks. A trader who understates a loss — through carelessness or wishful thinking — is far more dangerous than one who miscounts a gain. Firms test for it because risk systems depend on honest, accurate self-reporting under pressure.
Compute your P&L while the cards are still visible, not after. Buys settle at (sum − ask) × size, sells at (bid − sum) × size. Doing the subtraction during the reveal leaves only one number to remember.
Trade smaller until the arithmetic is automatic. The double penalty means an uncertain calculation on a losing trade is the most expensive situation in the drill.