How do you know if the prediction market you're trading is any good?
Here are two very different questions at about the same odds. What's holding each one up?
ExploreBy Oct 31, about the same odds.
Will Marine Le Pen win the 2027 French presidential election?
Will Apple be the largest company in the world by market cap on December 31?
Two examples we chose. Both trade on Polymarket; we're not affiliated. Oct 2 to Oct 31.
Did anyone actually trade?
About $25,000 on a typical day
About $2,400 on a typical day
Dollars traded each UTC day. Same scale.
| Day | Le Pen | Apple |
|---|---|---|
| Oct 2 | $29,112 | $3,184 |
| Oct 3 | $5,993 | $2,481 |
| Oct 4 | $23,622 | $265 |
| Oct 5 | $121,198 | $96 |
| Oct 6 | $11,591 | $3,898 |
| Oct 7 | $14,886 | $1,227 |
| Oct 8 | $25,434 | $3,743 |
| Oct 9 | $15,484 | $10,184 |
| Oct 10 | $5,830 | $585 |
| Oct 11 | $28,724 | $175 |
| Oct 12 | $12,470 | $4,783 |
| Oct 13 | $30,086 | $2,780 |
| Oct 14 | $29,635 | $4,263 |
| Oct 15 | $98,708 | $40,561 |
| Oct 16 | $24,062 | $2,259 |
| Oct 17 | $17,012 | $2,326 |
| Oct 18 | $25,956 | $1,219 |
| Oct 19 | $31,132 | $445 |
| Oct 20 | $14,323 | $1,231 |
| Oct 21 | $28,147 | $9,149 |
| Oct 22 | $54,181 | $13,648 |
| Oct 23 | $14,974 | $10,808 |
| Oct 24 | $12,849 | $324 |
| Oct 25 | $43,954 | $567 |
| Oct 26 | $10,939 | $587 |
| Oct 27 | $26,151 | $2,712 |
| Oct 28 | $25,457 | $1,759 |
| Oct 29 | $31,023 | $18,555 |
| Oct 30 | $32,306 | $7,932 |
| Oct 31 | $21,972 | $448 |
Who's trading?
A taker is someone who accepts a posted price. Some trade far more than others. Weighed by dollars, it's like this:
19 equal takers on Le Pen
5 on Apple
Oct 2 to Oct 31.
Where buyers meet sellers
Every market has two lines: buyers and sellers.
The gap is the distance between them.
Say you want to buy $1,000 of Yes right now, at posted prices.
You'd pay about $1 over the mid price on Le Pen. About $42 on Apple.
Le Pen
One price$1,000 filled
- 41.2¢$8
- 41.1¢$1,356
- 41.0¢$125
- 40.9¢$2
- 40.8¢$10
- 40.7¢$68
- 40.6¢$10
0.1¢ gap
mid price
- 40.5¢$4,297
- 40.4¢$4,565
Buyers (bids)
Apple
Seven prices$1,000 filled
- 45.8¢$316
- 45.7¢$0
- 45.5¢$0
- 45.4¢$0
- 45.1¢$0
- 45.0¢$0
- 44.9¢$0
2.3¢ gap
mid price
- 42.6¢$30
- 42.5¢$7
Buyers (bids)
Before fees. Order books at the last check on Oct 31, UTC, when the venue's fee was off. With the fee back on since Nov 1, add about $24 on Le Pen and $22 on Apple.
So how do you know if these markets are any good?
| Le Pen | Apple | |
|---|---|---|
| Traded on a typical day, Oct 2 to Oct 31 | $25,000 | $2,400 |
| Takers, weighed by dollars, Oct 2 to Oct 31 | 19 | 5 |
| Extra cost of $1,000 (Oct 31, before fees) | $1 | $42 |
Rounded.
The grade
Now measure every market the same way.
Every market graded as of Nov 1.
Fixed bands, so a market’s letter doesn’t depend on how other markets did.
Le Pen's market lands in B+. Apple's lands in F.
About the same odds. Very different markets.
B+
Le Pen market
Liquidity B+ · Integrity A
Belief Systems Market Grade · As of Nov 1, 2026 · Active
F
Apple market
Liquidity F · Integrity B
Belief Systems Market Grade · As of Nov 1, 2026 · Active
- Possible downgrade
- Possible upgrade
- Under review
- Near resolution
- Grade limit
- No defined resolution date
Belief Systems Market Grade · As of Nov 1, 2026 · Beta beliefsystems.xyz/grades
Grades use fixed criteria. A market’s grade does not depend on how other markets are graded. Markets with no grade as of Nov 1 aren’t shown.
As of Nov 1, 2026
See the recordThat letter is called a Market Grade.
- Liquidity:
- can you trade near the price, and at what cost?
- Integrity:
- how widely spread is the trading, and does the order book keep working?
Records update daily, so they can differ from this Nov 1 snapshot.
The numbers behind these grades
| Le Pen | Apple | |
|---|---|---|
| Cash within 5¢ of the price, at least 75% of the time, thinner side | $26,379 | $909 |
| Cost of a $1,000 trade, as a share of the Yes price, costlier direction, with the fee then in force (75th percentile) | 2.6% | ran past the 20 price levels we record |
| Effective takers, weighed by dollars | 19.2 | 5.3 |
| Steady daily trading (25th percentile) | $14,886 | $585 |
| Median daily trading | $25,445 | $2,404 |
| Time the book was broken | 0% | 0% |
At least a quarter of the time, in the costlier direction, $1,000 on Apple cost a fifth of the Yes price or more, or ran past the 20 price levels we record.
The venue's fee was off from Oct 20 until Nov 1, so the 30-day cost figures cover both periods.
Cost is relative to the price, so it reads higher at low prices, and Apple's price was low for much of the window.
These are the measures behind the grade. The Market Grade changes only through grade actions, so working the formula by hand can give a different letter on a given day. Liquidity and Integrity letters update daily.
Integrity is a screen, not proof that a market is free of manipulation.
Figures are rounded.
These are two examples we chose.
Every market on the grid
Look up any market.
All gradesHow the method works
Market Grades are in beta. Not affiliated with Polymarket. Market Grades measure market quality, not the likelihood of any outcome. Market Grades are unsolicited: no graded party requests, pays for, or previews a grade. Belief Systems holds no positions in graded markets. A grade is not a recommendation to buy, sell, or hold any position, and is not a substitute for a user's own assessment.