IEX vs. SIP Market Data
IEX is a single exchange feed; SIP data consolidates eligible U.S. exchange reports through the consolidated tapes.
Read the draftTrading Concepts
Understanding Bid, Ask, Last, and Spread
Bid and ask are quoted buying and selling prices from a particular feed; last is a reported executed trade. The spread is ask minus bid. These fields can disagree without any display error.
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Quotes and trades answer different questions
A bid of $100.00 and ask of $100.05 give a $0.05 quoted spread. A last trade of $100.03 may have occurred before either current quote was posted, or on a different eligible venue. Do not interpret last as an executable buying or selling price.
Feed and timestamp matter
An IEX-only quote is from one exchange, while SIP data consolidates eligible U.S. venue reports under its rules. A platform can therefore show a different best bid, ask, or last trade depending on feed, entitlement, timestamp, and trade-condition filters.
Read wide spreads carefully
Spreads can widen when trading is thin, especially outside regular hours. A wide displayed spread signals limited visible liquidity on that feed; it does not by itself measure the full cost of a trade. Market Watch 360 displays information and does not place orders.
Sources and further reading
- Alpaca Latest QuotesBid and ask quote fields and feed selection
- Alpaca Market Data FAQFeed coverage, entitlements, and recent historical data