The present study quantifies the bias induced in subjective video quality experiments by the semantic labels (e.g. “Bad”, ”Poor”, ”Fair”, ”Good”, and ”Excellent”) generally associated to the grading scale. In this respect, a common theoretical ground for subjective quality evaluation, encompassing both continuous and discrete scale evaluations is deployed. The experimental study is performed according to the ITU-R BT 500-11 and to the SSCQE (Single Stimulus Continuous Quality Evaluation) method. A total of 330 human observers are inquired. They are grouped in three main panels of 110 observers, on panel for each type of content under investigation (HD 3D TV, HD TV, low quality video). In order to grant statistical relevance, each main panel was split into three sub-panels, referred to as the reference (60 observers), validation (25 observers) and cross-checking (25 observers) panels. The experimental results demonstrate that the “Excellent” label has an important impact (biases up to 65% with respect to the a priori expected situation), independent with respect to the content under investigation. Large biases are also brought to light for other semantic labels; however, these seem not to be solely induced by the label semantic.