Hi,
I too was burned big time when the SET price came in so high. I sent emails to standardandpoors.com complaining, and they sent a file to me with all 500 stock opening prices that were used for the calculations. They also sent the formula, but it is vague and they wouldn't provide the exact formula.
I set about finding each stocks' opening price based on other data providers. Being a programmer, I wrote a small java app that pulled the prices from Yahoo, which I think can be relied upon. I then created a spreadsheet based upon this info. I would say that 80% were lower than the prices that Standard and Poors used. At first they seemed a small pct, but after totaling, averaging, etc. the discrepancies could be the reason for such a high SET price.
I found that the total of price discrepancies was 94.79, which is .05 % of SPX high price of 1883.98. If you take the difference between the SET and actual highest price of the SPX, you get 9.32, which is .0049 % of SPX high price of 1883.98. More than enough to be inaccurate. In fact 10 times higher.
I'm not sure what to do about this, but I had 100 1880/1885 call credit spreads working and I lost a lot of money. And considering Thursdays close, Friday's open and close, I believe that I and many others erroneously lost money.
If anyone would like the spreadsheet, send me a private message. I would like to pursue this further, especially with those who lost money because of this anomaly.
Most likely the only way to make this right is a lawsuit, but if it resulted in your account being made right, it would be well worth it. Besides, it is high time that this method of setting the closing price for SOME indexes be put out to pasture.