A method and apparatus for performing Quality-of-Service (QoS) calculations on packet-based communications networks using a QoS measure which is based on data included in non-lost packets, as well as on data included in lost packets, when the proper interpretation of the data in non-lost packets depends upon data in one of the lost packets. Two new QoS measures that address the limitations inherent in the prior art PLR (Packet Loss Rate) measure are introduced. The Packet Loss Distortion Rate (PLDR) measure determines both packets which are lost, as well as packets whose proper interpretation depends on one or more packets which have been lost. The Media Distortion Rate (MDR) measures the actual quantity of media material that is lost, regardless of how the material is grouped into individual packets.