I can't emphasize the fact enough that error-free data isn't possible. If you start to back test your trading theories with historical data, keep in mind that there are errors in almost all data. Personally, I use two sources of data and compare them before performing tests, but I have the luxury of some programming knowledge and access to several data sources.
If you're on a limited budget, compare charts from two sources or use free data from two sources and make sure everything matches up. If you spot discrepancies, find a third data source and see which of your first two is correct!
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