From Seven Digits to Seventy-Nine

In the late 1970s, at a university in the United States, two psychologists, William Chase and Anders Ericsson, were looking for a volunteer for a very dull experiment. They wanted to know how far a person could stretch the memory for digits. They found a student, known in their reports as S.F., who was a keen runner and an ordinary undergraduate.

In the first session he could repeat back about seven digits, which is what most people can do. The psychologists asked him to come back, a few hours a week, to practise.

S.F. practised for many weeks. He found that he could do better if he turned the digits into something meaningful. As a runner, he knew a lot about running times, so he made groups of digits into times: 3 4 9 2 was 3 minutes 49.2 seconds, which was almost a world-record mile. He also used dates and ages. He built the groups into larger groups, in a structure.

Slowly the number rose: ten digits, twenty, thirty, forty. After about two hundred and thirty hours of practice, spread over around a year and a half, he could repeat back about seventy-nine digits, from a single hearing. Nobody had thought it possible for an ordinary person.

The experiment taught the psychologists something about practice. S.F.'s memory itself had not changed: when they tested him on letters, he went back to about six or seven. What had grown was a skill, a set of methods tuned to digits. Skill grows with practice, in small regular steps.

The same is true for you. Everything you have learned in the first twenty-eight rooms is a skill. It will fade if you do not use it, and it will grow if you do. This lesson builds a plan for keeping it alive.