May marked the worst month in two years for the Nasdaq and the S&P 500. How did the industry group rankings respond? More than half the top 20 industry groups at the beginning of May were out of the top rankings by the end of the month. That's heavy turnover. At the start of May, groups representing four sectors held 13 of IBD's top 20 industry rankings. Retail and medical were the strongest, with four slots each. Construction and leisure were