Consider a stock that is really worth $100: when its price is above $100 at the beginning of a year it tends to fall the change ranges randomly between a rise of 10 percent and a fall of 20 percent, but when the price at the beginning of a year is $100 or less, it tends to rise the change ranges randomly between a fall of 10 percent and a rise of 20 percent. Which tends to do better over a ten-year period, a strategy of buying $5,000 worth of stock only when the price has fallen in the past year or one that invests the same total amount of money as a lump sum at the beginning of the period