Bid-ask spread
Why does a trader pay slightly more than the last price, and why can't candle data show that cost?
In plain words
The bid-ask spread is the gap between the highest price a buyer will pay and the lowest price a seller will accept at the same moment. Buying takes the ask, selling takes the bid, so a round trip costs the spread plus any fee.
Why it matters
The spread is an immediate, certain cost, unlike the uncertain price move ahead. It widens when liquidity thins or volatility rises, so small or fast-trading positions can lose a visible share of their edge before the market has moved at all.
In a real week: ETH, week of 23 March 2020
ETH's week of 23 March 2020 shows this blind spot directly. The candle record reports an open of 122.31, a close of 124.5, a high of 143.36 and a low of 119.5, a range of 19.97% and realised volatility of 103.0% a year, on dollar volume of 0.88bn — yet it contains no spread field at all. Two traders executing the same hour could face different costs.
A common mistake
Treating the last traded price in a candle as the price you can deal at. It records one completed trade; your own order may fill at an ask above or a bid below it.