In equivalent groups design, two equivalent tests are presented to two equivalent groups of testinees. The groups
could be randomly selected, and for this it is sometimes called random groups design. The advantage of this design
is that the problems of single-group design, like the effects of fatigue and exercise, can be reduced. Moreover
minimum time is needed and the test can be completed in one presentation. The defect of this design, however, is
that unknown levels of taking sides are entered in equating process, for the groups are not often exactly the same in
distribution of their abilities. In order to control the taking sides, greater samples are often needed in this design