Alternate from reliability

One way of avoiding the difficulties encountered in the test retest reliability is through the use of alternate from of the test, one consisting of similar items (with some difficulty level) but not the same items.
Alternate forms reliability compares two equivalent forms of a test that measure the same attributes. It is also known as parallel from or equivalent forms reliability.
Properties of Alternate-Forms
By administering an alternate form after a suitable interval following administration of the first form, a reliability coefficient reflecting errors of measurement due can be determined.
To control for the confounding effect or test form with administration time, Form A should be administered first to half the group and Form B to the other half; then on the second administration, the first group is given from B and the second group is given Form A.
The resulting correlation between scores on the two forms, referred to as a coefficient of stability and equivalence, takes in to account errors due to different administration times or different items.
Similarities with Test-Retest method
Both strategies involved two administrations to the same subjects with an intervening time interval.
Intervening changes in motivation and individual difference in amount of improvement would produce fluctuation in test scores and thereby somewhat reduce reliability estimates in both methods.
Difference with Test-Retest Method
The alternate forms methodology introduces item sampling difference as an additional score of error variance with time sampling.
Even though the two forms may be equally difficult on average, some subjects may find one form quite a bit harder than the other because supposedly alternate items are not equally familiar to every person.
Advantage
The method of alternate forms provides one of the most rigorous assessments of reliability commonly in use.
It provides a useful measure for evaluating many tests.
It is useful in follow-up studies or in investigations of the effect of some intervening experimental factor on test performance.
The use of several alternate forms also provides a means of reducing the possibility of coaching or cheating.
Limitations
There are lots of limitations such as-
It is often very difficult to develop two versions of a test that are truly parallel.
Alternate forms of a test are also quite expensive-nearly doubling the cost of publishing a test and putting it on the market.
For the above reasons, fewer and fewer tests are being released in this format.