The presentation slide inspired Reichheld to develop a new metric, earned growth rate, which measures the revenue growth generated by returning customers and their referrals. A related statistic, the earned growth ratio, is the ratio of earned growth to total growth.

Earned growth has two elements. The first is the back-for-more component captured by a battle-tested statistic called net revenue retention (NRR), which is used in several industries, most notably software-as-a-service (SaaS). Once you have organized revenues by customer, you can determine your NRR.
The second component is earned new customers (ENC). It is the percentage of spending from new customers you’ve earned through referrals (as opposed to bought through promotional channels).

Tracking the behaviors of customers tagged as earned versus bought will help you determine their relative lifetime value, illuminating which customer segments and acquisition channels represent the best investments.

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Article très intéressant qui revient sur les limites du NPS pour l'enrichir avec un 2ème KPI.