The U.S. government's IDEA, or Individuals with Disabilities Education Act, requires schools to test special education and learning disabled students every three years unless the parents and school district agree it's unnecessary. Mandatory re-testing helps educators give learning disabled students an appropriate education that meets their needs. IDEA guidelines cover students that have hearing, sight or speech and language impairment, and it also includes those with health or mobility issues, an emotional condition, autism, a brain injury or mental retardation.
Longitudinal research is the evaluation of a subject over a period of time. IDEA testing is one example of this since it requires educators to track the progress of special education students throughout their academic life. Longitudinal research is also a way to compare the results of schools at the local, state and national level. By giving IQ tests to sample groups in different school systems, researchers can track how schools are doing over time in relation to each other. They also can use this data to learn the strengths and weaknesses of individual programs and to make changes where needed.
Sometimes a parent or teacher requests a re-test if a student's test results are surprisingly low. Poor results are possible when a student has a bias against the test, has anxiety about the test or becomes preoccupied with other things. If a student doesn't seem to be trying and the student's score is worse than expected, it makes sense to deal with the problem and take the test again.
Life is unpredictable and it's possible for events surrounding a test to cast doubt on the results. You might have a fire or other emergency that disrupts the test, a student could get sick before, during or after it, a power failure might happen or students could get caught cheating. Any unexpected event that can impair a student's concentration is a possible reason for re-taking the test.