hey, over recent days I noticed for AAPL that best bid is often greater than best ask. Within each regional exchanges all is fine: bid<ask, but bests are bid>ask. Just wonder if this allows any arbitrage for market. This keeps for long time, I can see this for half an hour sometimes.

Is it before trading starts? Pre-open in many markets there are order imbalances - in europe an auction algorithm sorts it out.Or maybe this is the issue of specialists controlling the market so that you have to trade through them, and can't hit bids and lift offers in the market? That can be frustrating... in other words you see a really high bid, but you can't hit it, as the specialist only lets you hit his lower bid...

Tanya, I'd contact your quote vendor or IT dept -- someone should be able to confirm that your quotes are not stale. Also check the bidsize and asksize on the regional exchanges; e.g. if you can only make a nickle arbing 100 shares it's hardly worth the effort and trans cost.

This would occur before and after market. the market is in pre-open which allows bids and asks to be placed on either side of the market and then a "match " price is calculated based ont eh relevant bid and asks placed. This sometimes occurs when the market is open after a company announcement, where the exchange will place the companys trading into temporary halt while the market gets a chance to read the announcement and factor it into the current price. This is how it works on most markets and i assume that is what u are seeing. Any suckh arbitrage opportunities would not exist for more then a minute or two as there are so many peopel out there looking for arbitrage opportunities

One more case this may happen is in pure limit order markets, where the most aggressive order is not a market order but a limit order crossing the other side of the book. For example, if i see a stock quoted at 95-97 posting a limit buy for some thousand shares at say 105, will have as a result my order first to gain priority second to sweep all available depth at 97 both visible and hidden, then climb up one tick, say one cent, sweep all available depth there and so on, until the order gets totally filled. If by climbing up to 105 the order hasnt filled totally, then it remains there awaiting for all the incoming market orders to match at that price untill it gets totally filled. Things dont change unless someone posts another limit buy order at 106 or higher gaining both price and time priority. With such an order one can set an upper price limit i am willing to buy instantly. That said, observing such patterns may last a couple of minutes if not a few seconds. Its unlikely that such a pattern may happen in pure dealer markets because with such an action a dealer exposes his balls, moreover, usually there is an interdealer trading system where dealers can trade among themselves for inventory management reasons, recall they seek to go home flat.HTH

this situation (when best bid>best ask) happened all last week (apr11-15) when market WAS OPENED. for AAPL I saw this almost all the day, so it was not a quick glitch. Within regional exchanges- it was fine, for example one regional showed 35.91/35.93, another 35.94/35.95, but bests were 35.94/35.93. Sometimes I saw such negative spread up to 0.10$ ! And this could be monitored for quite long time (hlaf an hour). prices did change , but so that bid still was > ask. I checked this on different vendors, and all showed this, so this wasn't IT/vendor mistake. It looks like regionals for these stocks show very different bid/asks what leads to such situation. however not sure why this happens when market is open?

Who is your vender? Are you using their frontend or are you using a custom GUI that displays their data feed? Does your vender provide time & sales data? I am assuming you are not a trader because you could have submitted orders into the market to test this. Did you talk to the trader who trades AAPL and see what he had to say? I bet this is a bug either in your GUI or your data venders system.

I checked using Hyperfeed/ComStock and Active financial as well, they both showed the same data via GUI, so looks not GUI issue as different vendors showed the same things....

btw, you are right, I am not a trader, but analyst. will try to check with traders

an update for all qho participate in this duscussion - I got quite reasonable reply from AAPL trader "This is simply a reflection of fast markets and the fact that on the regional exchanges those prices are possibly for small "order book" orders.They may not be executable in the way that true electronic market prices are, i.e. those on the ISE."