In nearly 829 mid-size and large US firms, the percentage of minority managers decreased dramatically over a five year period after mandatory diversity training programs were implement. By contrast, voluntary training increased minority representation among managers. Most dramatic increases were achieved through mentoring programs, diversity task forces and having a full time diversity manager. 

Dobbin, Frank and Alexandra Kalev. “Why Diversity Programs Fail.” Harvard Business Review (July-August 2016 Issue).