What is a significant difficulty with estimating reliability using the alternate or parallel forms procedure?

The significant difficulty with estimating reliability using the alternate or parallel forms procedure primarily stems from the challenge of creating two sound instruments that are equivalent or parallel. In this method, two different but comparable tests are used to assess the same construct or ability in order to determine how consistent the results are across both forms.

The complexity arises from the need to ensure that both instruments measure the same underlying concepts with the same level of accuracy and relevance. If the tests differ significantly in content, structure, or difficulty, it can lead to unreliable results that do not truly reflect the reliability of the measurement. Thus, achieving equivalence in parallel forms is crucial for accurately estimating reliability, making this option the most significant difficulty in this reliability estimation method.