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# Bollinger Bands This method takes in an array of historical prices for a stock, a period (the number of days to calculate the moving average and standard deviation over), and the number of standard deviations to use for the upper and lower Bollinger Bands. It uses the `moving_averages` method to calculate the moving average for the given period, and then calculates the standard deviation of the closing prices for each window of the given period. Finally, it calculates the upper and lower Bollinger Bands based on the moving average and standard deviation, and returns an array containing the upper and lower bands. The `num_std_dev` parameter in the Bollinger Bands method specifies the number of standard deviations to use for the upper and lower bands. The default value for this parameter can depend on the specific security being analyzed and the time period being used. A common default value for `num_std_dev` is 2, which corresponds to the standard deviation of the price data over the given time period. Using a value of 2 for `num_std_dev` will result in the upper and lower bands being placed at a distance of two standard deviations from the moving average. However, the optimal value for `num_std_dev` can vary depending on the volatility of the security being analyzed. For highly volatile securities, a larger value for `num_std_dev` may be more appropriate, while for less volatile securities, a smaller value may be more appropriate. Ultimately, the best default value for `num_std_dev` will depend on the specific use case and should be chosen based on the characteristics of the security being analyzed and the preferences of the analyst. The difference between the upper and lower bands can be an indicator of how volatile the stock is.
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