A small bank in Tashkent has 1,200 customers. Last month its marketing team wanted to send a printed letter to every customer who had not yet tried either of the bank's two digital services: the mobile app and the telephone call centre.
The team had two lists. The app list showed 700 customers. The call centre list showed 650. A junior analyst added the two numbers, got 1,350, and reported that every customer already used at least one service. "We have more users than customers," he said proudly. "No letters needed."
His manager, Lola, laughed. The two lists overlap, she explained. A customer who uses both services appears on both lists and is counted twice. She asked the IT department for one more number: how many customers were on both lists. The answer was 250.
Now the calculation was simple. Seven hundred plus 650 is 1,350, but 250 of those people were counted twice, so only 1,100 different customers used at least one service. That left 100 customers who use neither. Those were the people who should receive the letter.
Lola went one step further. Of the 700 app users, 250 also called the centre, so 450 used only the app. Of the 650 callers, 400 used only the phone. The bank decided to send those 400 customers a short guide to the app, because they were already comfortable with the bank but were paying for every call in time.
The letters cost very little. The lesson, Lola told her team, was free: before you add two lists, ask who is on both.