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Nornagest comments on Accuracy Versus Winning - Less Wrong
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Well, you need some framework. You said that IQ points are not "necessarily a consistent metric for cognitive function". First, what is "cognitive function" and how do you want to measure it? If you have no alternate metrics then how do you know IQ points are inconsistent and what do you compare them to?
The usual answer is that it is measuring the g factor, the unobserved general-intelligence capability. It was originally formulated as the first principal component of the results of a variety of IQ tests. It is quantifiable (by IQ points) and it does have real-world consequences.
I don't understand what that means.
Saying that IQ measures g is like saying that flow through a mountain creek measures snowmelt. More of one generally means more of the other, but there's a bunch of fiddly little details (maybe someone's airlifting water onto a forest fire upstream, or filling their swimming pool) that add up to a substantial deviation -- and there are still a lot of unanswered questions about the way they relate to each other.
In any case, g is more a statement about the correlations between domain skills than the causes of intelligence or the shape of the ability curve. The existence of a g factor tells you that you can probably teach music more easily to someone who's good at math, but it doesn't tell you what to look for in a CT scan, or whether working memory, say, will scale linearly or geometrically or in some other way with IQ; those are separate questions.
g is an unobserved value, a scalar. It cannot say anything about "causes of intelligence" or shapes of curves. It doesn't aim to.
g was observed as a correlation between test scores. That is by definition a scalar value, but we don't know exactly how the underlying mechanism works or how it can be modeled; we just know that it's not very domain-specific. It's the underlying mechanism, not the correlation value, that I was referring to in the grandparent, and I'm pretty sure it's what ialdabaoth is referring to as well.
To be more precise, the existence of g was derived from observing the correlation of test scores.
Moreover, g itself is not the correlation, it is the unobservable underlying factor which we assume to cause the correlation.
It is still a scalar-valued characteristic of a person, not a mechanism.