Weighing evidence in decision systems

To combine items of evidence supporting a particular decision in a way which facilitates comparison between the cases supporting different conclusions, we advocate some improvements to a very simple method used in an experimental evidential reasoning system, relative evidential supports (RES). RES is very perspicacious in that decisions are made by trading off evidence items pro and con each contender. This makes the reasoning easy to follow by domain experts. The most important feature of this system is that it does not use numbers to indicate evidence strengths; simple comparisons of evidential supports for alternatives is the principle adopted. However, the de facto method of combining and comparing evidence is in a sense too stringent and in another sense too lax. We suggest some ways of improving the practical evidence comparison (or weighing) method of RES while retaining its conceptual characteristics and simplicity.