Where are historical prices and data coming from? I've noticed discrepancies on a regular basis. Example is Vanguard Long-Term Bond ETF (BLV) closing price on January 31, 2017. Market XLS is pulling in a closing price of $89.59, but Vanguard and Yahoo is showing a closing price of $89.63. This is not an issue everyday, however it is frequent enough to be concerned about accuracy. Another example is volume. Market XLS is pulling in a volume for the S&P 500 Index of 1,959,278,818 on January 31, 2017. Yahoo is showing volume of 4,087,450,000 which seems more accurate.