Professor John Dawes wrote an article titled Why Net Promoter Score is Actually a Bad Tool and What to Use Instead.
« In summary, if you want to know if your customers are satisfied, ask them about satisfaction. If you want to know about actual word of mouth recommendation, ask about that. But don’t ask ‘would you recommend’ – for all the reasons outlined above. Rather, ask ‘have your recommended that company to others in the past week or month’. That way you’ll understand how much actual positive word of mouth is occurring for your brand. »