Price belongs to quantity
In a fictional book, sellers offer 20 securities at RUB 102, 30 at 103 and another 10 at 106. A buyer’s order for 60 securities, assuming the offers remain unchanged, has three execution prices. The cost is 20 × 102 + 30 × 103 + 10 × 106 = RUB 6,190. The average price is approximately RUB 103.1667. It is neither the best ask nor the simple average of the three displayed prices: weights come from the actual quantity at each level.
Relative to the best ask, this average is approximately 1.1438% higher. The comparison is explicit: execution of 60 securities is compared with buying the entire amount at the initial best offer. It is not a complete trading-cost measure; commission, market movement, taxes and other participants’ reactions are excluded. Calling every such difference “slippage” without naming the base makes otherwise valid calculations incomparable.
Spread, depth and resilience
The spread is the distance between the nearest bid and ask. Depth is the available quantity at different price levels. Resilience can be discussed through how offers return after trades or news, but one snapshot cannot measure it reliably. SEC materials treat the bid–ask spread as a cost factor for an exchange-traded fund alongside other secondary-market features. A narrow spread for a small quantity does not imply the same terms for a much larger position.
Historical trading turnover answers another question. It shows how many trades occurred during a selected period, not how much can be executed now at a given price. Large turnover yesterday does not require yesterday’s participants to post matching orders today. For some bonds, trade frequency, available sizes and information transparency differ; FINRA lists liquidity among important debt-instrument factors. Do not transfer one venue, session or asset class’s measure automatically to another.
What changes the arithmetic?
If the seller cancels ten securities at the last level, the original calculation no longer describes full execution. If a new offer appears at the same time, the result can change in the other direction. Processing order, hidden size and special regimes cannot be reconstructed from one image. The teaching model freezes the book to separate arithmetic from market dynamics; it does not assert that a particular order will execute at the listed levels or that visible size is available entirely to one participant.
To check another calculation, list the price, quantity and cash amount for each execution. Divide total cash by total quantity, and list costs outside the price separately. Clarify whether the units are shares or lots, whether [accrued interest](/guides/accrued-interest/) is included and whether timestamps match. If the result is compared with the last trade, midpoint or best offer, name the comparison base. The [order-to-settlement route](/guides/exchange-and-settlement/) helps distinguish an estimate from a confirmed result.
What does the average price really test?
Replace the total quantity in the example with only the amount at the first level. The average then equals the best offer, assuming it remains available. This is not a contradiction: the size of the task changed. Therefore, saying that an instrument is “liquid” without naming volume, time and acceptable price difference is incomplete. The same book can be sufficient for a small order and insufficient for a large one.
If only total seller quantity is known but its distribution across levels is hidden, the average price cannot be reconstructed. The same quantity may be concentrated near the best offer or much farther away. Quantity sufficiency and execution cost must be checked separately. A chart of completed trades cannot be inserted into a live-offer table without explanation. A verifiable task needs levels, weights and a time snapshot; everything else is an assumption.