In summary, these results imply that, though the effect is perhaps small and difficult to detect, the perceived fidelity of an audio recording and playback chain is affected by operating beyond conventional consumer oriented levels. Furthermore, though the causes are still unknown, this perceived effect can be confirmed with a variety of statistical approaches and it can be greatly improved through training.
Implications for experimental design
1. Training - Test subjects should be trained in how to
discriminate, given examples and informed of their results in practice sessions
before the test.
Implications for meta-analysis
We also uncovered an issue with the use of standard statistical hypothesis testing applied to multiple trials with dichotomous outcomes. This issue, which occurred in many studies, may lead to Type II errors, and to our knowledge has not been widely addressed elsewhere in the literature.
Future research directions
There is a strong need for several listening tests. First, it is important that all test results be published. Notably, there is still a potential for reporting bias. That is, smaller studies that did not show an ability to discriminate high resolution content may not have been published. Second, it would be interesting to perform a subjective evaluation incorporating all of the design choices that, while not yielding Type I errors, were taken in those studies with the strongest discrimination results, e.g., Theiss 1997 had test subjects blindfolded to eliminate any visual distraction. If these procedures are followed, one might find that the ability to discriminate high resolution content is even higher than any reported study. Finally, no research group has mirrored the test design of another team, so there is need for an experiment that would provide independent verification of some of the more high profile or interesting reported results.
Many studies, reviewed in Section 1, involved indirect discrimination of high resolution audio, or focused on the limits of perceptual resolution. These studies were not included in the meta-analysis in order to limit our investigation to those studies focused on related questions of high interest, and amenable to systematic analysis. Further analysis should consider these additional listening tests. Such tests might offer insight both on causes of high resolution audio perception and on good test design, and might allow us to provide stronger results in some aspects of the meta-analysis.
However, many of these additional studies resulted in data that do not fit any of the standard forms for meta-analysis. Research is required for the development of statistical techniques that either transform the data into a more standard form, or establish a means of meta-analysis based on the acquired data. Finally, further research into statistical hypothesis testing of (multiple comparisons of) multiple trials with dichotomous outcomes would be useful for interpreting the results of many studies described herein, and widely applicable to other research.
Additional data and analysis is available from https://code.soundsoftware.ac.uk/projects/hi-res-meta-analysis.
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