Stocks ended mixed in slow trade Friday as the market greeted Federal Reserve chief Janet Yellen’s speech with a yawn. The Nasdaq tacked on 0.1%, while the S&P 500 retreated 0.2%. The IBD 50 strode 0.5% higher. Volume fell across the board. Yellen’s speech at the annual conference in Jackson Hole, Wyo., repeated themes that she had emphasized before. She said that the recent drop in the unemployment rate overstated the labor market’s health.