Item Response Theory (IRT)
A statistical framework for understanding how test-takers respond to individual items based on their ability level. Modern alternative to classical test theory.
IRT models how a person's latent trait (like Extraversion) affects the probability they'll answer an item a certain way. Unlike classical scoring (just counting right answers), IRT accounts for item difficulty and discrimination.
Key principle: items themselves vary in difficulty and usefulness. A question like "Do you enjoy parties?" is easier (less discriminating) than "Do you organize large group events?" IRT uses this variation to create more precise scores.
IRT advantages: shorter tests without losing precision (you need fewer, better items), adaptive testing (computer adjusts difficulty based on responses), and better item analysis. Modern personality tests increasingly use IRT instead of classical scoring.
Source: Lord & Novick (1968), foundational psychometrics text.