It’s about customers’ willingness to recommend a product or service to someone else aka "Referral via Word of Mouth"
The percentage of customers who were enthusiastic enough to refer a friend or colleague - one of the strongest sign of customer loyalty, correlated directly with differences in growth rates among competitors.
Customers who not only return to rent again but also recommend your products or services to their friends, correlate tightly with profits or growth for your business. No surprise, judging from this nielsen survey which shows which type of advertising is the most influential of all.
If you are doing survey about customer satisfaction, It turned out that a single survey question can, in fact, serve as a useful predictor of growth. Just don't forget to ask this
"How likely you are going to recommend our product or service to someone else?"
