PowerShares and Dorsey Wright have launched their fourth exchange traded fund that uses a relative-strength strategy. PowerShares DWA SmallCap Technical Leaders Portfolio (DWAS) picks 200 stocks from a universe of the 2,000 smallest U.S. companies in a broader set of 3,000 of the largest U.S. listed stocks. Dorsey uses a proprietary method to rank stocks based on relative strength. Relative strength measures how a stock performed vs. the market