The least-squares prediction line in Figure 5.1 goes through the middle of the cloud of points, representing the mean values for the heights of fathers and sons, but does not follow the diagonal line of ‘equality’. It is clearly lower than the line of equality for fathers who are taller than average, and higher than the line of equality for fathers who are shorter than average. This means that tall fathers tend to have sons who are slightly shorter than them, while shorter fathers have slightly taller sons. Galton called this ‘regression to mediocrity’, whereas now it is known as regression to
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