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There is a phenomenon in the historical data of the S&P 500 index, of extended periods with repeated occurrences of 'today's open = yesterday's close'.

It could have made sense had it been the case all the time up to a certain date, and then changed due to managerial decision or improved technology, however, this thing comes and goes like an occasional wave. What is that?

It looks like this:

enter image description here

(data from Yahoo! Finance)

Below is a list of these occurrences per year. SPY included for comparison.

enter image description here

In particular 2004-5, it happened almost all throughout both years! (and yet almost, not all.)

The most recent 'surge' was in 2012-13.

The Dow Jones and Nasdaq Composite do not exhibit this 'feature'. S&P 100, on the other hand, does. Which leads to believe that it has something to do with Standard & Poor's.

If someone could shed light on this mystery...

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  • $\begingroup$ to take a step back, why is this problematic to you? it could be for any number of reasons: stale data, exchange traded on, etc. not to mention data quality on yahoo finance is widely known to be far from perfect...you get what you pay for. $\endgroup$
    – Chris
    Commented Jul 10, 2019 at 22:35
  • $\begingroup$ There is no mystery... it's incorrect. Use a different source. $\endgroup$
    – amdopt
    Commented Jul 11, 2019 at 0:05
  • $\begingroup$ The open should not be equal to the previous close as most of exchanges have pre-trade and prices could even jump. $\endgroup$
    – Vitomir
    Commented Jul 11, 2019 at 7:48
  • $\begingroup$ Thanks for the comments guys. I've considered the (in)accuracy of the data and compared with a few sources online, it was the same. Although who knows if they don't take the data from Yahoo Finance as well... could you direct me to a website, online charting platform, anything that uses data not from Yahoo Finance? Just to compare. It is not about problematic or not, just to understand the behavior of the index so that I can use the data for statistical analysis. $\endgroup$
    – Arigato
    Commented Jul 11, 2019 at 9:05
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    $\begingroup$ UPDATE for future reference: you were right unanimously. Yahoo Finance data is no good. It feels silly to have spent time analyzing it. I have learned a lesson here. Thank you for your answers. $\endgroup$
    – Arigato
    Commented Jul 18, 2019 at 7:51

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