Editing Mean Opinion Score (section)

== Modern Challenges and Limitations ==

=== Statistical and Methodological Issues ===

MOS faces several inherent limitations that researchers and practitioners must consider:

'''Ordinal Scale Problems''': MOS ratings are based on ordinal scales where the ranking of items is known but intervals between ratings are not necessarily equal. Mathematically, calculating an arithmetic mean from ordinal data is problematic, and median values would be more appropriate. However, the practice of using arithmetic means is widely accepted and standardized.<ref>https://en.wikipedia.org/wiki/Mean_opinion_score</ref>

'''Range-Equalization Bias''': Test subjects tend to use the full rating scale during an experiment, making scores relative to the range of quality present in the test rather than absolute measures of quality. This prevents direct comparison of MOS scores from different experiments.

'''Contextual Dependence''': MOS values are influenced by the testing context, participant demographics, and the presence of anchor stimuli (very high or low quality samples that influence perception of other stimuli).

=== Scalability and Cost ===

Traditional MOS testing is time-consuming and expensive, requiring recruitment of human evaluators and controlled testing environments. This has led to increased interest in:

* Crowdsourcing platforms for distributed evaluation
* Objective quality models that predict MOS
* Automated evaluation metrics that correlate with human perception

=== Limitations in Advanced Applications ===

As technology advances, particularly in AI-generated content, traditional MOS evaluation faces new challenges:

'''Ceiling Effects''': When synthetic speech approaches human quality, MOS becomes less discriminative, with most systems scoring in the 4.0-4.5 range where small differences may not be statistically significant.

'''Missing Dimensions''': MOS provides only an overall quality rating and may miss specific aspects like speaker similarity, emotional expression, or intelligibility of specific linguistic phenomena.

'''Cultural and Linguistic Bias''': MOS scores can vary based on evaluator demographics, language background, and cultural factors, potentially limiting generalizability across diverse user populations.