  Mean Opinion Score (MOS) is a popular measure for evaluating synthesized
speech. However, the scores obtained in MOS tests are heavily dependent upon
many contextual factors. One such factor is the overall range of quality of the
samples presented in the test -- listeners tend to try to use the entire range
of scoring options available to them regardless of this, a phenomenon which is
known as range-equalizing bias. In this paper, we systematically investigate
the effects of range-equalizing bias on MOS tests for synthesized speech by
conducting a series of listening tests in which we progressively "zoom in" on a
smaller number of systems in the higher-quality range. This allows us to better
understand and quantify the effects of range-equalizing bias in MOS tests.
