NPS
Net Promoter Score is a metric used to evaluate customer loyalty. The results of this survey are usually given on a scale of 1-10, where 9-10 means customers are ready to recommend the company, 6-8 expresses an indifferent, passive attitude, and 1-6 indicates that survey participants are opposed to recommending the company to others and are closer to criticizing it.
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Useful links:
https://pl.wikipedia.org/wiki/Net_Promoter_Score
http://6ix.pl/pg/102/nps-net-promoter-score---filar-rekomendacji
https://www.quirks.com/articles/7-nps-boosters-we-hope-you-never-use
https://strategex.com/top-5-tips-for-improving-your-nps-score/