Since I invest in mutual funds rather than individual stocks, I'm the wrong person to ask. However, I'd look at how often you end up closing a position because of the stop loss before it's had a chance to grow. Also, is keep following the stock value for those stocks you get out of for a few months after to see how often that stop loss stopped a eventually larger loss and how often it has been a small fluctuation that prevented you from sticking with an eventual winner.