(FORTUNE Magazine) – Nexis, our favorite computerized information-retrieval system and leading resource for settling 1 A.M. living room arguments, has lately been getting a new kind of workout. With the opening of Congress, your servant has been | fascinatedly fiddling with a Nexis program called Billcast. It offers odds on the passage of any bill in Congress. The current total is around 6,000, the great majority of which would just make things worse if passed, but that is not Billcast's problem, only the country's. More precisely, the system offers percentage probabilities of each bill's making it over four legislative hurdles. Thus, the clean-air bill introduced by House commerce committee chairman and renowned powerhouse John D. Dingell of Michigan has a 69% chance of getting through Dingell's own committee, a 57% chance of being passed by the House, a 47% chance of making it through committee in the Senate, and a 45% chance on the Senate floor. The forecasting system generating these odds was originally developed by ''public choice'' economists at George Mason University. The public-choice school aims to explain government behavior by elaborating the interests and incentives of government officials (including legislators), and Billcast is very much in this tradition. Its forecasts reflect a view that congressional action cannot be entirely explained by the solons' ideology and the voters' economic interests. Also needing to be factored into the equation are certain institutional requirements of Congress itself. The Billcast model of congressional behavior emphasizes, for example, the leadership's need to reward ''team players''; in any case, the model's database shows that Congressmen who accept discipline and reliably pay off their logrolling debts are the ones whose bills get passed. Bills sponsored by committee chairmen also, unsurprisingly, do better than average. Professor W. Mark Crain of George Mason, a major designer of Billcast, says that the model undergoes some ad hoc tinkering to reflect the batting averages of individual Congressmen. Owing to his recent Wade Boggs-like average, the ratings have been raised for all bills introduced by John P. Murtha, a publicity-shy Democrat based in Johnstown, Pennsylvania, who is described in the Almanac of American Politics as a ''silent, behind-the-scenes power.'' Whether Murtha's bills make things better or worse is, to be sure, another question.