# adjusted close prices on SP500

When I look at the adjusted close prices of SP500, for example, I notice that the numbers are always significantly below the actual closing.

In the explanation of what adjusted prices are, one gets the impression that the Open prices for the stock on the next day that a certain event happens (such as splits or dividend pays) should be equal, or very similar to, the adjusted close price.

However, this is never the case. Why is there a systematic discrepancy?

Example:

Date,Open,High,Low,Close,Volume,Adj Close
2009-09-30,106.36,106.46,104.62,105.59,254383000,97.27
2009-09-29,106.51,107.02,105.78,106.00,133733900,97.65
2009-09-28,104.85,106.55,104.83,106.32,118285800,97.94
2009-09-25,104.78,105.36,104.09,104.45,204059000,96.22
2009-09-24,106.41,106.64,104.55,105.01,228636800,96.73


I'm trying to evaluate a certain automated strategy but this difference is giving me a hard time to know what my position is after a trade is done. I understand that I buy and sell at the raw prices, but it's not clear to me how to properly evaluate the value of the portfolio at a given point in time.

Thanks.

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Adjust closes gets asked on here a lot [ 1, 2, 3, 4, 5, 6, 7 ]. Do any of these answer your question? –  chrisaycock Oct 2 '13 at 17:22
I had a look at them before asking the question and I am afraid it didn't help. –  elelias Oct 2 '13 at 18:05

Adjusted series are cumulative of all events that happen after the date. Meaning, if you look at the prices on 2009-09-30, the adjusted close represents all corporate actions that happen after this date. This will create a cumulative effect causing adjusted prices farther back in time to deviate more and more from the actual prices observed that day.

If you look at a more recent price series and find one that has only had one corporate action you'll see a price series that more accurately reflects what you're expecting. Here is data from IBM showing from Yahoo to demonstrate:

Date        Open    High    Low     Close   Volume      Adj Close
Aug 8, 2013 189.45  189.87  186.79  187.93  4,547,600   187.93
Aug 7, 2013 189.60  189.93  188.05  188.56  3,901,700   188.56
Aug 7, 2013 0.95 Dividend
Aug 6, 2013 192.26  192.51  190.27  190.99  5,938,500   190.04
Aug 5, 2013 195.16  195.88  194.35  195.50  2,490,900   194.53
Aug 2, 2013 195.50  195.50  193.22  195.16  3,874,000   194.19


On Aug 6, 2013 IBM closed at 190.99, went ex-dividend for 0.95, then opened the next day at 189.60. The Adjusted close for Aug 6, 2013 was 190.04 representing the actual close 190.99 less the dividend.

Note, that the open price on the day after a corporate action does not (and likely won't) be equal to the previous days close net of the adjustment. Other factors such as overnight news will affect the next days open.

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wow, that really cleared things up for me, thanks so much. So the adjusted close from the entire history can potentially change every day. –  elelias Oct 2 '13 at 20:43
Just think of it this way: the adjustment value for date D is simply sum of all adjustment values from now until D. –  Louis Marascio Oct 2 '13 at 21:38