Por Luísa Canto e Castro Loura (Direção-Geral de Estatísticas da Educação e Ciência, Portugal).

In Education, the main random variable is, in general, the knowledge-level of the student on a certain subject. This is generally considered to be the result of a process that involves one or more educational agent (e.g. teachers) and the reflection and training of the student. There are many covariates including: innate capacities, resiliency, perseverance, family and health conditions, and quality of education.
While being a subject that all of us has some a priori knowledge (essential in statistical modelling), it has an initial difficulty that may lead to significant bias and wrong interpretations: the measurement of the main random variable. From Educational psychometrics, measurement tools are the tests or examinations and the main variable is a latent variable.