We present a new characterization of propositional formulas called entropy, which approximates the freedom we have in assigning the variables. Like several other such measures (e.g., back-door and back-door-key variables), it is computationally expensive to compute. Nevertheless, for small and medium-size satisfiable formulas, it enables us to study the effect of this freedom on the impact of various SAT heuristics, following up on a recent study by C. Oh (Oh, SAT'15, LNCS 9340, 307-323). Oh's findings were that the expected success of various heuristics depends on whether the input formula is satisfiable or not. With entropy, and also with the measure of solution density, we are able to refine these findings for the case of satisfiable formulas. Specifically, we found empirically that satisfiable formulas with small entropy "behave" similarly to unsatisfiable formulas.
Keywords: SAT; entropy; solution-density.