A client device receives streamed encoded content data, such as encoded video data, which has been encoded at a constant perceptual quality. Several different versions of the content are available to be streamed to the device, at different perceptual quality levels. In order to decide which quality level to request from a content server at intervals the device calculates the delivery rates that would be required for each level of quality. The calculation of the delivery levels is made in dependence on whether the actual delivery rate that has been received so far is greater than or less than a constant bit rate delivery schedule that decreases monotonically but which guarantees to deliver the encoded data in a timely manner such that no buffer underflow and interruption of reproduction takes place. The monotonically decreasing bit rate schedule is defined by one or more “critical points”, which are the points at which the bit rates are decreased, and correspond to the points where the delivery schedule is just equal to the decoding schedule of the encoded data. At such points there would be no encoded data in a buffer at the client. If the delivery rate has been greater than the constant bit rate schedule for a quality then the delivery rate required going forward for a particular quality level can be calculated from the critical point information, and no more complicated calculation is required.