Item Information

In Item Response Theory, Item Information quantifies the precision with which a given item measures a test-taker’s latent ability, $\theta$.

In Classical Test Theory, we assume that items are equally precise across all test-takers. In contrast, IRT measures precision as a function of ability. And item information tells you where along a continuum of ability a given item functions best (i.e. where it provides the most information).

Item Information is inversely related to standard error.

Item information is also additive, so we can get test information by just summing all of the item information scores, i.e.:

$$I_{test}(\theta )=\sum _{i=1}^N{I}_i(\theta )$$