Unexpectedly Intriguing!
July 13, 2015

How volatile are stock prices?

To answer that question, we tapped Yahoo! Finance's entire history of the S&P 500's open, high, low and closing stock price values for each trading day going back to 3 January 1950. We then calculated the percentage change of each trading day's closing value with respect to the previous day's closing value. Our results are presented below, along with the basic statistics we calculated:

S&P 500 Daily Volatility (Percent Change Between Closing Value and Previous Trading Day's Closing Value), 3 January 1950 - 6 July 2015

For the 16,481 trading days worth of closing values we considered, the mean percentage change from one day's closing value to the next is slightly positive at 0.03%. Through 6 July 2015, the S&P 500 has closed above its previous day's closing value some 8,724 times (52.9%) and closed below its previous day's closing value 7,633 times (46.3%), while closing at the same value 124 times (0.8%).

And that 1,091 difference in the number of up days versus down days explains why the value of the S&P 500 index has increased from $16.66 to $2,068.76 in the sixty-five and a half years since 3 January 1950.

Data Source

Yahoo! Finance. S&P 500 (^GSPC) Historical Prices, 3 January 1950 through 6 July 2015. [Online Database]. Accessed 6 July 2015.

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