In response to an unknown trigger, the immune system may begin producing antibodies that instead of fighting infections, attack the body's own tissues. Treatment for autoimmune diseases generally focuses on reducing immune system activity.
What's taught in school. Some of the items in standardized achievement tests measure the knowledge or skills that students learn in school. In certain subject areas, such as mathematics, children learn in school most of what they know about a subject.
The statistic used to measure the validity is the same as the one to measure reliability, a correlation or Cohen's alpha . What is different is the types of scores being compared. To measure reliability, two (or more) sets of scores from the same individuals are taken using the same operational definition.