Please refer to the following articles:
Jorion, Philippe, Value at Risk, 2nd edition, McGraw-Hill, 2001
Lopez, Jose A., 2000, An Academic Perspective on Backtesting and Stress-Testing Presentation for Credit Risk Models and the Future of Capital Management, Federal Reserve Bank of San Francisco, http://www.frbsf.org/economic-research/files/lopezbktesting.pdf
Wikiperia, Value at Risk, https://en.wikipedia.org/wiki/Value_at_risk