http://en.wikipedia.org/wiki/Backtesting http://www.investopedia.com/terms/b/backtesting.asp read this one carefully before you backtest: http://www.priceactionlab.com/Blog/2010/10/proper-use-of-back-testing/
It depends on the strategy. This year we have seen trending markets, range bound markets, sideways moving markets, major sentiment changes, cycle reversals, momentum burst, a few other scenarios, and last but not least a major correction. Its good to look at as much data as possible when back testing but this year alone has been good by it self to back test based on almost every way the market can move has moved this year alone. Good luck back testing and Generating Alpha (Returns in excess of the current periods market movement) The market is providing many opprotunities
I scalp Asian session.In this case i should only avoid major corrections,wich happens 2-4 times a year,right? I mean nothing is left to backtest
You do not have to avoid a major correct if your on the right side of the market. But scalping the Asian session during a range bound market is Smart, very smart Good luck