StockCharts goes to great lengths to make sure the data is accurate. Once the stock markets close, the daily quote (open, high, low, close and volume) for each stock is audited by the exchanges. That means that the people at the exchange go back and review all of the trades for each stock - tossing out any incorrect numbers - and then reissue the “official” values for open, high, low, close and volume for that stock. In order to make sure we have the “official” numbers, we recollect the daily data from our data vendors several times each day after the market closes. Our goal is to have “official” numbers in our database for every stock's daily values.
Intraday data, however, is a different story. Exchanges do not audit intraday data values or issue corrections, nor are there any other public sources where intraday data can be cross-checked; the data simply is what it is. Intraday data is essentially a running record of what our data feed told us the stock was trading at during each minute of the trading day. That said, we still work hard to fix intraday data once it gets off the datafeed and into our databases (which happens after the markets close each day). Once the data is “ours,” we can go in and fix any blatantly obvious problems, such as spikes and dropouts.
If you come across a bad data point on an intraday or daily chart, we ask that you wait until the audit is complete, typically by 6:00 PM Eastern Time, before reporting the data error. This is because we are unable to correct bad data while the markets are open and must ourselves wait for the audit process to be completed.