So there's a 7 point discount today between the Dec ES and SPX. Let's assume 0% interest. If all the S&P stocks that were going to pay out dividends between now and the Dec ES futures expiration went ex on Monday, then that 7 point discount would go away on Monday (ignoring interest and some minor second order effects) The 7 point discount seems to go away proportionally because those dividends are paid out proportionally (and obviously there is some interest which does behave proportionally), but not exactly. Some weeks a lot of stocks in the index go ex, some weeks not many. As a result, that delta between ES and SPX changes in a more lumpy fashion that just dividing 7 by the number of trading days remaining.
I know how to calculate this effect, it's just a lot of work for me to develop the screen scraper to find ex dates and dividend amounts for component stocks and plug them into a formula based on those stock's component weights. I had hoped someone had a calculator online somewhere so I wasn't an idiot reinventing the wheel, but couldn't find one using my friend Google. It's sounding like it doesn't exist, so maybe I'll build it and put it up for anyone who wants it.
Well, a few things here. Dividends do change, often in fact. So that uncertainty has to be factored in. Second, when a dividend is paid out, are you properly accounting for the fact that you can "re-invest" that cash flow back into the S&P? So let's call SPX cash. If one is selling SPX, they are losing this benefit. They need to ask a higher price to be compensated for the loss in cash flow. This higher price will be reflected in the difference between SPX and ES. One way to evaluate this is to set up the arbs and see where you lose money. Long ES/Short SPX. Long SPX/Short ES. Yes, there is a borrowing cost for SPX which right now is zero or close to it. So using this theory ES should trade lower then SPX. If every stock went ex-dividend tomorrow this benefits the SPX holder! Why? Cash flows today are worth a lot more then then cash flows 3 months from now. Hence there should be a premium to SPX over ES.