Net Promoter 3.0 – Harvard Business Review by Fred Reichheld 
As its popularity grew, NPS started to be gamed and misused in ways that hurt its credibility. Unaudited, self-reported Net Promoter Scores undermined the usefulness of NPS. Over time its creator, Fred Reichheld, realized that the only way to correct this problem was to introduce a hard, complementary metric that drew on accounting results.