Measuring a Person

The expertise board is about what practice changes in a person. This lesson is the other half of that picture: what already differs between two people before the practice starts, how anyone measures it, and what the measurements can and cannot say.

Before you start. How Psychology Knows What It Knows showed that a correlation is two things moving together, not one causing the other, and that a claim is only as good as its replication. Learning & Expertise showed that practice reshapes a mind. Here you need only one more idea, refreshed now: a correlation runs from 0 (no link) to 1 (a perfect link), so 0.5 means “they tend to go together, with plenty of exceptions”.

The idea in 30 seconds

The first widely used intelligence test, from Paris in 1905, was built to find children who needed help, and its author warned against reading the score as a fixed quantity.
Every mental test correlates positively with every other, and the common factor, g, comes out nearly identical whichever test battery you use.
Measured ability predicts education strongly (about .56), job status moderately (.43) and income weakly (.20), and its link to job performance was cut from .51 to about .31 on reanalysis.
Personality is measured on five mostly independent dimensions, the Big Five, found by grouping the words a language uses for people, and a person’s rank on them grows steadier through youth and then levels off.
Heritability is the share of the differences between people in one population that tracks genes. It is not the share of one person’s trait that comes from genes, and it changes when the environment changes.

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