Let's say it's an index-tracking ETF. Isn't the ETF price determined by the index value? Why does it have bid/ask spread then? Could someone explain the bid/ask concept when trading ETFs?
The price of ETF is determined by index value. This is generally calculated after the market closes.
When the market is open if enough people holding ETF believe markets will go up, they will ask more. There are a bunch who believe it will go down or not go as high as others think and will bid low.
This results in a spread. As no one can predict the exact value the ETF will end on close of markets, the ETF behaves similar to stock, the only difference being it will not deviate too much from the actual value.
Isn't the ETF price determined by the index value?
Technically, the price is determined by the market's view of the ETF's assets, which is supposed to track some index. It consists of numerous stocks The price does not necessarily have to match the index exactly.
Take SPY as an example. It's current value is 264.15, while the S&P 500 sits as 2647.60, so not quite a 1:10 ratio. If you compare the two on a finance site, you'll see that the normalized graphs are very close, but do have some deviation. In fact, SPY posts a tracking error of 0.05, which means that on average, the ETF price is about 0.05 above or below the actual index value.
Why does it have bid/ask spread then?
Like any other stock, the bid/ask spread is determined by the limit orders for that index, with buy limit orders filling the "bid" queue and sell limit orders filling the "ask" queue.