On method: the retention multiple

Every volume of Principles of Practice ends by naming the numbers its field repeats most often and explaining why the book does not use them. Customer success has several and they share a shape.

It costs five times more to acquire a customer than to keep one. A five percent lift in retention raises profit by twenty five to ninety five percent. Existing customers are sixty to seventy percent likely to buy again, new ones five to twenty. Each is repeated constantly, each is presented as a general law, and each either has no traceable source or has one that was narrower than the claim made from it.

The third of those is the clearest case. It circulates with no citation at all, and the ranges are so wide that any outcome confirms it.

It is worth being precise about the objection. The underlying claim, that retention is high leverage, is very probably true and this entire volume assumes it. What is not acceptable is treating a number of unknown provenance as a constant of business and sizing programmes with it.

So this volume argues from mechanism and from your own arithmetic: the reason mix, the time to churn measured on your own churned accounts, and a holdout to establish what your programme actually does. Those give a number that is true about you.

The four questions from Volume I apply unchanged. Who produced it and what were they selling. What was the sample and of what population. What exactly was measured, in units. Has anyone independent reproduced it.