the expected value, and will tend to become closer as more trials are performed. Even the most stupid system can be very profitable for a day or two but of course it fails miserably over a long period of time. According to to its definition Law of large numbers is a theorem that describes the result of performing the same experiment a large number of times. Go long rule: the trend went straight up the previous day (Close1-Open1 0) and the price retraces a certain percent of previous High previous Low.
I need to know how many pips per day a certain pair moves. If it makes 13,000 trades during 13 years without profits, then its not a good system. If the equity curve points straight up then its the first sign of curve fitting, thats why I like ugly looking equity curves clearly showing the drawdown period. Statistical principles and methods are invaluable tools in forex, ignore them and get ready to fail. If it makes 13,000 trades and the profit doubles (Im not mentioning anything about drawdown here it means that it made X during one year and X during 12 years, a very unequal distribution of profits. It survives but its curve fitted for a single market aspect only. For example, lets say we have a system that makes 1,000 trades per year. If you toss a coin 10 times, anything can happen, you may even get 10 heads or 10 tails in a row even if the overall probability is 50 because the number of trials is simply too short and statistically not significant. In forex we are not dealing with certitudes, we are only dealing with probabilities.