Item Response Theory: How Smart ARE You?

Psychometrics is all about measuring the psychological characteristics of people; for example, scholastic aptitude.  How is this done?  Tests, of course!  But there's a chicken-and-egg problem here: you need to know both how hard a test is, and how smart the test-taker is, in order to get the results you want.  How to solve this problem, one equation with two unknowns?  Item response theory--the data science behind such tests and the GRE.

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