  We establish a random variable substitution lemma and use it to investigate
the role of refinement layer in multiple description coding, which clarifies
the relationship among several existing achievable multiple description
rate-distortion regions. Specifically, it is shown that the El Gamal-Cover
(EGC) region is equivalent to the EGC* region (an antecedent version of the EGC
region) while the Venkataramani-Kramer-Goyal (VKG) region (when specialized to
the 2-description case) is equivalent to the Zhang-Berger (ZB) region.
Moreover, we prove that for multiple description coding with individual and
hierarchical distortion constraints, the number of layers in the VKG scheme can
be significantly reduced when only certain weighted sum rates are concerned.
The role of refinement layer in scalable coding (a special case of multiple
description coding) is also studied.
