They are often used for placement and admission decisions by schools or universities. They also can assist schools' determination of which students may be in need of a special education service. Another use for these tests is in the selection process for certain jobs. Conversely, aptitude tests do not evaluate knowledge, but predict a student's future performance. The most common aptitude test is the SAT, which is administered to most of the senior high school class each year, and is used by colleges to collaborate in admission decisions.
Reliability refers to the test giving consistent results over the years.
Assuming that intelligence does not change for any reason over a period of time, an intelligence test should be constant too. It involves having people to take the test twice, the second time separated from the first one with a brief period of time. If the results are comparable, then the test is reliable. The split-half method, consists of splitting the tests into two halves, and making two different tests with them. Then we can compare the results from part one and part two.