  For the quantification of QoE, subjects often provide individual rating
scores on certain rating scales which are then aggregated into Mean Opinion
Scores (MOS). From the observed sample data, the expected value is to be
estimated. While the sample average only provides a point estimator, confidence
intervals (CI) are an interval estimate which contains the desired expected
value with a given confidence level. In subjective studies, the number of
subjects performing the test is typically small, especially in lab
environments. The used rating scales are bounded and often discrete like the
5-point ACR rating scale. Therefore, we review statistical approaches in the
literature for their applicability in the QoE domain for MOS interval
estimation (instead of having only a point estimator, which is the MOS). We
provide a conservative estimator based on the SOS hypothesis and binomial
distributions and compare its performance (CI width, outlier ratio of CI
violating the rating scale bounds) and coverage probability with well known CI
estimators. We show that the provided CI estimator works very well in practice
for MOS interval estimators, while the commonly used studentized CIs suffer
from a positive outlier ratio, i.e., CIs beyond the bounds of the rating scale.
As an alternative, bootstrapping, i.e., random sampling of the subjective
ratings with replacement, is an efficient CI estimator leading to typically
smaller CIs, but lower coverage than the proposed estimator.
