“There's a tool called the word of mouth coefficient that lets you quantify offline word of mouth by looking at how many new users you're getting through channels like direct, branded search, or social in a given period. Subtract your returning users plus new users from non-organic channels — and that coefficient tells you how efficiently you're getting new users through word of mouth from your existing base.”

Word of mouth coefficient: quantify offline referrals by subtracting all known acquisition channels

NPS measures intent to recommend but can't tell you how many actual new users result. The word-of-mouth coefficient backs into a measurable estimate by treating anything that can't be attributed to known channels as WOM-generated. Carter used this at Quizlet to spot seasonal WOM spikes (back-to-school, exam periods) and COVID-driven country divergences — enabling proactive investment in virality during high-coefficient windows.