Modern item response theory aims at modeling the relationship between latent traits and observed responses to test items. It focuses on measuring traits that are not directly observable by using observable responses. The latent trait to be measured can be a mental ability, a proficiency or skill but also an attitude, an emotional state, physical fitness, or other “constructs”. It does not necessarily refer to persons but can also refer to properties of other entities as software when software reliability is to be investigated or to institutions concerning their efficiency in performing. Achieving reliable measurement often requires a substantial number of items. Measurement, in the context of IRT, is not an end in itself but typically serves a specific purpose, whether it be decision-making, theory testing, evaluation, or monitoring.