So I am trying out my new account with IB and find that their S&P 500 index quote (their symbol SPX) is somewhat different than what I get from TDAmeritrade and others. For example, today's close is 921.23 on TDA, WSJ, yahoo etc. But IB's quote shows 920.16. And the discrepancy is seen through out the day. The difference is pretty small (typically < 0.2) but sometimes it can make a difference between a new high or not which is important in my trading. TDA seems to update their SPX every 14 second or so while IB updates it much more frequently. On the other hand, Nasdaq composite (COMP) seem to match between the two. What gives? Are there multiple versions of SP500 ticks? I read that IB coalesaces stock quotes to speed things up but I doubt that they are doing it to slow moving index like SP500. Obviously they are not doing it for COMP.