Following my post "Rolling with the punches" here I received a comment (which is posted up on that post), from João in which he talks about his system having less than 20% winners, yet those winning trades are more than 4.5 times larger than his losers. Another trader I correspond with has a forex system that is working great with less than 25% winners.
These two traders are only the tip of the iceberg, but both their systems are exhibiting a positive expectancy. It also demonstrates how you can be wrong more than 3 times out of 4, yet still make money! That takes a lot of belief and confidence in their method, as well as strong risk control. It also comes back to the question of would you rather been proven right about the markets, or would you rather make money? Click here for a post I came across today nicely sums this up. In addition, think about the following quote from George Soros: "It's not whether you're right or wrong that's important, but how much money you make when you're right and how much you lose when you're wrong."
Remember that those win rates are an average, and do not take into account the fluctuations where a losing streak longer than that average could significantly erode capital - this is factored in when determining the risk per trade levels.
These kinds of statistics are typical of trend following systems and the individual parameters used - generally the tighter the initial stop, then the higher the percentage of losing trades. However, when a winning trade does take hold, then the R-multiple is many times larger. All summed up as cutting losses and letting profits run.