Active mutual-fund managers say you can’t judge their performance against passive benchmarks over short time periods. Is a decade and a half long enough? Over the 15 years ending December 2016, 95.4% of U.S. mid-cap funds, 93.2% of U.S. small-cap funds and 92.2% of U.S. large-cap funds trailed their respective benchmarks, according to the latest S&P […]