Peer assessment plays an important role in engineering education for an active involvement in the assessment process, developing autonomy, enhancing reflection, and understanding of how to achieve the learning outcomes. Peer assessment uncertainty for oral presentation skills as part of the FYP assessment is studied. Validity and reliability for complex assessments were conceptualised in the context of quantification, hierarchical system of uncertainties, aleatory uncertainty, and epistemic uncertainty. Systems as assessment methods were also conceptualised to define elements (i.e. examiner, examinee, and difficulty) as variables or/and constants, and to study the uncertainty behaviour of the peer assessment within the framework of the parametric uncertainty model. As a result, a singularity, at which the hierarchical structure of uncertainties breaks down, and its related behaviour were theoretically predicted and experimentally verified. Also, a validity paradox, when the "difficulty" level is sufficiently lowered, was found to be possible and verified for oral presentation skills. It may be useful for understanding fundamental characteristics of the peer assessment in which epistemic uncertainties are dependent on how the assessment is close to the singularity point.