In video coding, it is commonly accepted that the encoding paramaters such as the quantization step-size have an influence on the perceived quality. When dealing with Scalable Video Coding (SVC), the parameters used to encode each layer logically have an influence on the overall perceived quality. It is also commonly accepted that using given encoding parameters, the perceived quality does not change significantly according to the encoded source content. In this paper, we evaluate the impact of both SVC coding artifacts and source contents on the quality perceived by human observers. We exploit the outcomes of two subjective experiments designed and conducted under standard conditions in order to provide reliable results. The two experiments are aligned on a common scale using a set of shared processed video sequences, resulting in a database containing the subjective scores for 60 different sources combined with 20 SVC scenarios. We analyse the performance of several source descriptors in modeling the relative behaviour of a given source content when compared to the average of other source contents.