Many investors mistakenly establish an investment goal of achieving a specific rate of return. Setting a goal, unfortunately, does not make that return achievable. Indeed, no matter what the goal, it may be out of reach. Stating that you want to earn, say, 15 percent a year, does not tell you a thing about how to achieve it. Investment returns are not a direct function of how long or hard you work or how much you wish to earn. A ditch digger can work an hour of overtime for extra pay, and a piece worker earns more the more he or she produces. An investor cannot decide to think harder or put in overtime in order to achieve a higher return. All an investor can do is follow a consistently disciplined and rigorous approach; over time the returns will come.
– Seth Klarman, Margin Of Safety