In image quality assessments, the results of subjective evaluation experiments that use the double-stimulus impairment scale (DSIS) method are often expressed in terms of the mean opinion score (MOS), which is the average score of all subjects for each test condition. Some MOS values are used to derive image quality criteria, and it has been assumed that it is preferable to perform tests with non-expert subjects rather than with experts. In this study, we analyze the results of several subjective evaluation experiments using the DSIS method. Our first contribution is to discuss the statistical meaning of the MOS values, which has not been previously addressed in the literature. Second, our results show that, contrary to the established belief, there are advantages when performing subjective tests with experts, in that they allow experiments to be performed with fewer subjects, and to better determine the lower threshold of image quality.