IB shows the close price for SPX is 881.42, so today is up 0.04%. Yahoo Finance shows the close price for SPX is 879.56, so today is down. http://finance.yahoo.com/q/hp?s=^GSPC How do I make sense of this difference? Thanks a lot!
Go with the Yahoo numbers. IB seems to be always off with their quotes. It shouldn't affect your trading that much, at least it haven't for me personally.
The problem is the trading is done in IB and that's real money... so I should follow IB number, right?
check the timestamp of the quotes. IB continues to update spx around the clock (during after hours, i believe it is estimated from fair cash value of es futures) during regular market hours, the quotes should all be the same.
IB does update SPX around the clock. But even during regular hours, I find it very close but somewhat different than the quote from other sources (Yahoo, TDA, WSJ etc). For some reason, IB's quote is more frequently updated than the usual 15 second update you see with Yahoo and others. Yahoo's quote is the official version, I believe. CME seems to use Yahoo's closing SPX for its nightly futures report.