MUSHRA stands for Multiple Stimuli with Hidden Reference and Anchor and is a methodology for conducting a codec listening test to evaluate the perceived quality of the output from lossy audio compression algorithms. It is defined by ITU-R recommendation BS.1534-3.[1] The MUSHRA methodology is recommended for assessing "intermediate audio quality". For very small audio impairments, Recommendation ITU-R BS.1116-3 (ABC/HR) is recommended instead.
The main advantage over the mean opinion score (MOS) methodology (which serves a similar purpose) is that MUSHRA requires fewer participants to obtain statistically significant results.[citation needed] This is because all codecs are presented at the same time, on the same samples, so that a paired t-test or a repeated measures analysis of variance can be used for statistical analysis. Also, the 0–100 scale used by MUSHRA makes it possible to rate very small differences.
In MUSHRA, the listener is presented with the reference (labeled as such), a certain number of test samples, a hidden version of the reference and one or more anchors. The recommendation specifies that a low-range and a mid-range anchor should be included in the test signals. These are typically a 7 kHz and a 3.5 kHz low-pass version of the reference. The purpose of the anchors is to calibrate the scale so that minor artifacts are not unduly penalized. This is particularly important when comparing or pooling results from different labs.