The interpretative model has a central role in determining entropy. The qualifier "for a given set of macroscopic variables" above has deep implications when two observers use different sets of macroscopic variables. For example, let's consider observer A using variables U , V , W and observer B using variables U , V , W , X . If observer B changes variable X , then observer A will see a violation of the second law of thermodynamics, since he does not posses information about variable X and its influence on the system. In other words, one must choose a complete set of macroscopic variables to describe the system, i.e. every independent parameter that may change during experiment.