The third reason is continuity of test. Multiple tests are used to measure the growth or change of an ability or
quality in different levels of ability. In this situation, tests are often deliberately set with different levels of
difficulty. In order to measure this growth or change in ability, we can set different tests in different forms with
different levels of difficulty. Thus, when the difficulty of test is changed, the ability of measuring growth or change
cannot be guarantied. Therefore, these tests should be again based on the same scale. According to two first reasons,
equating of test is categorized under the name of horizontal equating, and according to the last reason is called
vertical equating