Links
[1] https://onlinecourses.science.psu.edu/stat500/lesson/1/1.5/1.5.1
[2] https://onlinecourses.science.psu.edu/stat500/lesson/1/1.5/1.5.3
[3] https://onlinecourses.science.psu.edu/stat500/lesson/9/9.1/9.1.2
[4] http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:standard_deviation_volatility
[5] http://adamhgrimes.com/how-do-you-calculate-volatility-in-excel/