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One can always see on the news for example, the movement of stock indexes.

Say for example: S&P +5.15 (0.05%) or S&P -0.57 (0.001%)

But how would I calculate the 2 numbers above? (+5.15 and 0.05%)?

For example, I checked on google finance and found for the S&P 500

Oct 22, 2012

Open 1,433.21

Close 1,433.82

Oct 23, 2012

Open 1,433.74

Close 1,413.11

So for the change in the S&P 500 for October 23rd, do I take the difference between the closing prices of Oct 22nd and Oct 23rd? Or the difference between the opening and closing price of Oct 23rd?

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The return from one day to the next is based on the Day's closing price.

To be clear - opening prices can be quite different from the prior day close. In your example, they are pretty close, but this is not always the case. Just pull a larger data set to observe this.

The above aside, dividends are not reflected in the index, so, after a dividend has occurred, you'd need to account for this if you are looking for true total return. In 2011, the S&P closed at 1257.60 vs a 2010 year end 1257.64. The return, however was 2.11%, not zero, after accounting for the dividends. To me, articles that suggest the yearly return was zero are inaccurate and misleading.

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  • Thanks @JoeTaxpayer , really helpful and I appreciate the point about the dividends -- didnt realise about that earlier! Just one thing, how would I calculate that the dividend return was 2.11%?
    – Richard80
    Commented Oct 28, 2012 at 18:20
  • Pulling data for all S&P 500 dividends is an onerous task. I'd suggest following an ETF such as SPY or VOO, to get really close to this number. Just add the expense to year end result for true S&P return.
    – JTP - Apologise to Monica ♦
    Commented Oct 28, 2012 at 23:12
  • Accounting for dividends is easy - just use the right index. In this case, the S&P 500 Total Return Index assumes dividends reinvested. It is a simulation and assumes that dividends are reinvested at the time of entitlement to the dividend rather than at payment date.
    – Richard at NorgateData
    Commented Aug 31, 2015 at 10:45

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