Originally posted by vikana
I guess I'm not getting it. How is the fair value for a futures contract related to the trading any particular stock?
Well, it's the theory that all boats float in the same water, so when a wave comes along, it tends to lift all boats... or when the S&P futures are up, most stocks tend to go up.
Now this is not always the case, as there are certain sectors that don't generally go with the S&P futures (ie airlines). But it's still a good rule of thumb.
An excellent example of this is GE. GE has a weighting that is about 7% of the S&P 500 (last time I checked). So if the S&P futures go up it's almost positive that GE will go up.
There are a few things driving this phenomena. One is the arbitrage people. Large players will play off the difference between the S&P futures and the stocks that are in the S&P. When there is a difference in price that is less than the fair value, they will buy "buy the spread" because there is almost 100% certainty that the difference will return to fair value at which time, they can "sell the spread" and lock in the profit. THey do the same thing on the short side. The use the S&P futures as the leader and buy/sell the stocks accordingly.
So, if you have a stock that is in the S&P 500 and there isn't anything "unusual" going on with that stock (rumors, earnings, industry news), it should float with the futures. If the futures go up, it should follow up the same percentage and vice-versa on the short side. So, if you see the spread increase significantly past fair value (which is the amount where the spread is at equilibrium), it's highly likely that your stock will go up. This is because the arb guys need to buy all the stocks so that they don't lose their shirt on the arb.
This also works with other futures/indices. For example, oil futures and the oil index, oil stocks... gold futures and the gold index, stocks... OEX, Russell 2000, etc.
So why do other stocks go up if they aren't in the S&P 500? Well, they are probably in a group and the stock (or stocks) with the largest market or capitalization in that group are probably in the S&P 500. So, when a group's leader goes up, other stocks in the group tend to go up. So, S&P futures go up, they lift the stocks in the S&P, the stocks in the S&P lift their respective groups, and your stock goes up. If it doesn't that's giving you some very important information.