The Probability of a Good Day

If you know a golfer's score on day one and are asked to predict his score on day two, your best guess must be that it will be average. The pattern is called regression to the mean because an extremely good score suggests a very lucky day. A few of the golfers who scored 66 on day one will do even better on the second day if their luck improves. Most will do worse, because their luck will no longer be above average. Arrange the players by their performance on day two and look at their performance onday one. You will find precisely the same pattern of regression to themean.