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How do I calculate two standard deviations away from the stock price?
Standard Deviation is a mathematical term that is useful in many areas. It can be considered a measure of how tight the data points are to the average. If there is consistency in the measurement system, then a point that is two standard from the average can be considered an outlier. It doesn't even need to be time based. We can say that a child born weighing less than X pounds is more than 2 standard deviations below the average.  Using it to look at a price or an index value doesn't make a lot of sense because many expect there to be long term growth. For example you would never say that you will buy a new jacket for your child when they are two standard deviations above their average height. You can say you will buy or sell a financial instrument when the P/E ratio is 2 standard deviations from some average. It could be the average of the long term history, or the index, or the sector. You could do the same thing for earnings per share or many other business of financial statistics. A standard deviation calculated from a time series assumes that the measurement will normally stay withing some bounds. And that straying from those bounds is a sign of the right time to buy or sell.