Net Promoter Score (NPS) measures customer loyalty by asking how likely people are to recommend a brand, on a scale from 0 to 10. Respondents are grouped into promoters (9 to 10), passives (7 to 8) and detractors (0 to 6). NPS is the percentage of promoters minus the percentage of detractors.
It is most useful as a trend over time, combined with open comments that explain the score.