Parallel Forms Reliability (Equivalent Forms)

Reliability and Validity > Parallel Forms Reliability

What is Parallel Forms Reliability?

Test-retest reliability
Parallel forms reliability can help you test constructs.

Parallel forms reliability (also called equivalent forms reliability) uses one set of questions divided into two equivalent sets (“forms”), where both sets contain questions that measure the same construct, knowledge or skill. The two sets of questions are given to the same sample of people within a short period of time and an estimate of reliability is calculated from the two sets.

Put simply, you’re trying to find out if test A measures the same thing as test B. In other words you want to know if test scores stay the same when you use different instruments.

Example: you want to find the reliability for a test of mathematics comprehension, so you create a set of 100 questions that measure that construct. You randomly split the questions into two sets of 50 (set A and set B), and administer those questions to the same group of students a week apart.

  • Step 1: Give test A to a group of 50 students on a Monday.
  • Step 2: Give test B to the same group of students that Friday.
  • Step 3: Correlate the scores from test A and test B.

In order to call the forms “parallel”, the observed score must have the same mean and variances. If the tests are merely different versions (without the “sameness” of observed scores), they are called alternate forms.

Similarity to Split-Half Reliability

Parallel forms and split-half reliability are similar, but with parallel forms, the same students take test A and then take test B. With split-half reliability, one group of students is split into two and both groups sit the test at the same time. The two tests in parallel forms reliability are equivalent and can be used independently of each other.

Advantages and Disadvantages


  • Parallel forms reliability can avoid some problems inherent with test-resting.


  • You have to create a large number of questions that measure the same construct.
  • Proving that the two test versions are equivalent (parallel) can be a challenge.

Henchy, Alexandra Marie, “REVIEW AND EVALUATION OF RELIABILITY GENERALIZATION RESEARCH” (2013). Theses and Dissertations–Educational, School, and Counseling Psychology. Paper 5.

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