d Abstract) Vladimiro Sassone1 , Karl Krukow2 , and Mogens Nielsen2 1 ECS, University of Southampton 2 BRICS , University of Aarhus We define a mathematical measure for the quantitative comparison of probabilistic computational trust systems, and use it to compare a well-known class of algorithms based on the so-called beta model. The main novelty is that our approach is formal, rather than based on experimental simulation.