---
title: "Reading the book, not the last print"
dek: "The quoted price is what one contract traded at. What you can fill in your size is a different number."
description: "Every screener shows a last price. It is the least useful number on the screen once you are trading in size. Depth and spread decide what your position actually costs."
author: "Kosmos Research"
author_role: "Quant desk"
date: 2026-06-12
category: "Explainer"
tags: ["Microstructure", "Order book", "Execution"]
reading_time_minutes: 5
canonical: https://www.kosmos.fyi/blog/reading-the-book-not-the-last-print
---

# Reading the book, not the last print

*The quoted price is what one contract traded at. What you can fill in your size is a different number.*

Kosmos Research · Quant desk · June 12, 2026 · Explainer · 5 min read

A market showing 63¢ is telling you what happened, not what is available. ==The number you can act on is somewhere in the book==: how much is resting at 63, how much at 64, and how far you have to walk before your order is filled.

## Three numbers that beat the last print

- **The spread.** The distance between the best bid and the best offer is your immediate cost of being wrong about direction. On thin contracts it dwarfs the edge people think they have.
- **The depth.** How much size sits at each level. A tight spread on one contract of depth is a quoted fiction.
- **The effective price at your size.** Walk the book to the quantity you actually want and average it. That is your real entry, and it is the only one worth putting in a spreadsheet.

| Metric | Value | Note |
| --- | --- | --- |
| Last print | history | one lot, some time ago |
| Top of book | an offer | for a specific size |
| Effective fill | your price | depth-weighted, after fees |

## Why thin markets punish conviction

The contracts with the largest apparent edge are usually the ones nobody is quoting. That is not a coincidence: the spread is wide because participants are unsure, and being unsure is precisely the condition under which you get filled by someone who knows more than you.

> **A useful habit**: Before sizing, ask what the position costs to exit, not just to enter. A market you can get into and not out of is a bet on your own patience as much as on the event.

None of this argues against trading illiquid contracts. It argues for [pricing the illiquidity into the edge](/blog/positive-expected-value-prediction-markets) before you decide the edge exists.