The paper deals with an influence of network jitter on effective packet loss in dejitter buffer. We analyze behavior of jitter buffers with and without packet reordering capability and quantify the additional packet loss caused by packets dropped in buffer on top of the measured network packet loss. We propose substitution of packet loss parameterPpl in ITU-T E-Model by effective packet loss Pplef incorporating network jitter, a jitter buffer size and a packet size as additional input parameters for E-Model.