Reading a Market

A prediction market page shows a cluster of numbers at once. There is a price, a bid and an ask beneath it, a depth ladder, a running volume figure, and a chart of where the price has been. Each answers a different question, and reading them together tells a trader something the headline price alone does not. Prices and Probabilities explains what the price means, and Order Books explains where it comes from. This guide is about the read itself, the practical work of standing in front of a live market and deciding, from its numbers, whether a contract is worth trading and how to enter if it is.

What do the bid, ask, and spread tell you on a prediction market?

Three numbers sit at the top of every market, and they are the first thing to read. The bid is the highest price anyone currently offers to buy at, and the ask is the lowest anyone offers to sell at. The price shown in large type is usually neither of those. Polymarket, for example, displays the midpoint between the best bid and the best ask, a convention its documentation describes. A venue that shows the last traded price instead is reporting the most recent match rather than a price anyone currently offers.

The practical consequence is that the headline price is a summary rather than a quote you can act on. Buying fills at the ask and selling fills at the bid, so a market displaying 34¢ with a 33¢ bid and a 35¢ ask lets you buy at 35¢ or sell at 33¢, and never at 34¢. The instrument underneath is an event contract that settles at $1 or $0, and Prices and Probabilities works through why the executable price is the one to compare your own estimate against.

The gap between the bid and the ask is the spread, and it carries two readings at once. The first is cost. A trader who buys and immediately sells, with nothing else changing, pays the full spread on the round trip, 2¢ in the example above, before any fees. The second is confidence. A spread of a cent or two means either side can transact near the displayed price, a sign that buyers and sellers have converged on a narrow range. A spread of eight or ten cents means they have not, and the displayed midpoint is a looser estimate than it appears. The cost side has been measured directly. One analysis of tens of millions of exchange trades found that traders who accepted standing prices for immediate execution earned systematically less than the traders whose orders they filled, with the size of the gap varying widely by market category (The Microstructure of Wealth Transfer in Prediction Markets).

When should you use a market order or a limit order on a prediction market?

Once you have read the spread, the next decision is whether to pay it, and the choice of order type is how that decision is expressed. A market order accepts the best price the book currently offers and fills right away. A limit order names the price you are willing to accept and waits in the book until the market reaches it. The two orders trade one certainty for another. A market order gives you certainty of filling and leaves the price uncertain. A limit order gives you certainty of price and leaves the fill uncertain.

The two roles named in the table, taker and maker, describe what an order does to the book. A market order takes liquidity that was resting there. A limit order rests and becomes liquidity for someone else to take. Order Books covers how the matching works underneath. For the read, the point is that a market order buys immediacy, and immediacy is exactly what the microstructure study above found takers paying for. When a market is deep and the spread is a cent, that cost is trivial and a market order is usually the sensible choice. When the spread is wide, a limit order posted inside it can let a patient trader capture part of the spread rather than pay it, at the risk that the market moves before anyone fills the order. The size of that risk depends on what sits behind the top of the book, which is the next thing to read.

How do you read depth and spot a thin prediction market?

Depth is how many contracts rest at each price level beyond the best bid and ask. The top-of-book numbers describe the surface of a market. Depth describes what lies underneath, and it is the difference between a price you can trade at scale and a price that exists only for the first small order.

Consider two markets that both display 34¢ with the same 2¢ spread. In the first, a few thousand contracts rest within a cent or two of the top on each side, so an order for a couple of hundred contracts barely moves the price and fills at or near 35¢. In the second, the book looks like this:

The best bid and ask are identical to the first market, and so is the displayed 34¢. Only 50 contracts sit at the 35¢ ask, though, and the next sell orders wait five cents higher. A market order to buy 200 contracts takes the 50 available at 35¢ and then jumps to 40¢ for the remaining 150, for an average fill near 38.75¢. That difference between the price you expected and the price you paid is slippage, and here a single ordinary order pays almost four cents of it. The same order in the first market would have cost a fraction of that.

Depth is also uneven across the price range. On venues built around an automated market maker (a formula that prices trades against a pooled inventory rather than a book of orders), liquidity is deepest near 50¢ and thins toward the ends by design (Paradigm's pm-AMM research examines why). On order book venues, depth is simply whatever traders have chosen to post, so it varies from one market to the next and from minute to minute. Venue Types compares the two structures. Either way, the read before sizing a position is the same. Ask whether the book can absorb your order without moving the price against you. For small size in a deep market the answer is yes and depth barely matters. For a larger order, or any order in a thin market, depth is the whole story.

What do volume and open interest tell you about a prediction market?

Volume and open interest both measure activity, and they measure different things. Volume counts how many contracts have changed hands over a window of time, whether a day or the whole life of the market. Open interest counts how many contracts are currently held as open positions.

Reading the two together separates a market where capital is genuinely committed from a market that is merely busy, and the pairing matters because each number can mislead on its own.

Two cautions keep volume honest. The first is that a cumulative figure is not current activity. A market can carry an impressive since-inception number and have gone quiet days ago, so read volume over the window you actually care about rather than the headline total. The second is that reported volume can be inflated outright. Wash trading, where a trader trades with themselves to manufacture the appearance of activity, can be common enough to distort the numbers. A network analysis of three years of Polymarket data flagged roughly a quarter of all volume as wash trading, rising above half of weekly volume at times and past 90% in some sports and election weeks, with the motive appearing to be incentive farming rather than profit (wash trading in decentralized prediction markets). Even honest reporting can overstate real trading, because naive counts fold in the creation and destruction of contracts. One decomposition of a heavily traded 2024 market found that roughly $958 million of reported monthly volume corresponded to about $391 million of actual trading (the anatomy of a blockchain prediction market). Market Manipulation covers how these patterns are constructed.

Open interest tends to resist that kind of inflation, because holding a position ties up capital that could be working elsewhere. A market with meaningful open interest has real money committed to it right now, which often reads as a steadier signal of genuine interest than a volume figure whose window and integrity you cannot see. The practical habit is to use volume to ask whether a market is currently active and its price is fresh, use open interest to gauge how much real conviction is parked in it, and treat any single volume number with suspicion until you know its window and its venue.

What does a prediction market's price history tell you about momentum?

The chart adds the dimension the top-of-book numbers leave out, which is time. A contract at 34¢ that traded near 20¢ a week ago is a different situation from a contract at 34¢ that traded near 50¢ a week ago, even though the two look identical on the surface. Three things are worth reading off the history: the level, meaning where the price sits now; the trajectory, meaning the direction and pace it has been moving; and the volatility, meaning how smooth or jagged the path has been.

Volatility is not random across the price range, and knowing its shape keeps a reader from mistaking ordinary churn for a signal. A structural study of a large Kalshi panel found that binary-market volatility tends to be highest when prices are near 50/50, rises as resolution approaches, and varies by category with how information arrives, so that economic-data contracts tend to drift smoothly while sports contracts move in jumps (Volatility in Prediction Markets). A price bouncing several cents a day near 50¢ on a sports market may be entirely normal, while the same movement on a long-dated economic contract would be worth a closer look.

Order Books covers the mechanics of why prices move at all. For the read, the useful stance is to use history for calibration. It tells you what ordinary movement looks like for this particular market, and whether the current price is resting at a stable level or sitting in the middle of a swing. What it cannot tell you on its own is whether the latest move was information or noise, and reading a move as agreement with your own thesis is one of the easier ways to talk yourself into a weak position.

What are the red flags to check before trading a prediction market?

Some patterns should make a reader pause, size down, or pass entirely. Most of them are failures of liquidity or integrity rather than of price, which means the displayed number can look perfectly reasonable while the market underneath it is not one you want to trade. These are the ones that recur:

• A tight spread over a thin book. The mirage from the depth section, where a few contracts at the best prices hide a near-empty ladder behind them.
• Stale quotes and no recent trades. A price that has not updated in a long while reflects a few old resting orders rather than a live consensus, and it can lurch the moment anyone arrives.
• Volume you cannot source. A large cumulative figure with little recent activity, or a market whose volume may be inflated, tells you less than it appears to. Check the window and the venue.
• A lopsided book. Far more size resting on one side than the other can precede a move, or can mean a market maker has stepped away and left the book unbalanced.
• An ultra-short horizon, especially in crypto. Five-minute up-or-down contracts tend to be among the hardest for a retail trader to approach.
• Ambiguous resolution rules. If the criteria are vague, the contract can settle against the obvious real-world outcome regardless of the price.

The short-horizon warning deserves its own note, because the structure of these contracts works against the casual trader. A research lab classified roughly 86% of taker dollars in short-horizon crypto markets as coming from bot-like wallets (Crypto on the Clock), and a separate study documented traders manipulating the underlying asset around settlement to extract roughly $8.2 million from participants in Polymarket's five-minute Bitcoin contracts, an effect the same study found absent in fifteen-minute contracts (Settlement Manipulation in Prediction Markets). The reader arriving to take a position in that window is often the one being selected against.

Two subtler flags are worth keeping in mind. A large study of Polymarket trades documented a tendency to overtrade the Yes or default option, so the Yes side of a market may carry a slight premium from that preference alone (accuracy, skill, and bias on Polymarket), which is worth remembering when a Yes price looks slightly rich. And resolution risk is independent of everything above. A contract settles on its written rules, not on what seems to have happened, so a market whose criteria are open to dispute is a hazard no amount of liquidity offsets. Market Resolution covers how that process works and where it goes wrong.

What should you check on a prediction market before you trade?

The individual reads combine into a short pre-trade routine. Each row below pairs the number to check with the sign that it is healthy and the sign that it is not, and the point of the routine is to reach a trade-or-pass decision before your own view of the event pulls you in.

The checks run in a rough order, because the early ones can end the process before the later ones matter. Liquidity comes first, since a market you cannot enter and exit cleanly is not worth analyzing further. Only once a market clears that gate is it worth weighing volume, history, and the resolution rules against your own read of the event.

None of these numbers tells you whether you are right about the event. They tell you whether the market is one you can trade cleanly and how much the entry will cost, which is a separate question from whether the price is wrong. Your edge, if you have one, is your own estimate set against the price, a comparison Prices and Probabilities develops and Market Accuracy puts in context. The market's own numbers are one input to that estimate. The public conversation around the event is another, and reading the two together, through tools such as sentiment analysis, can flag the moments when a price and the attention surrounding it have started to disagree.

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