Zeus is a computer program that takes a football decision -- for instance, punt or go for it on fourth down -- and simulates the game to completion as many as 1 million times, using years of data from NFL games and the characteristics of the competing teams.
The results are expressed in the option's "Game Winning Chance," or GWC. If a running play has a 2 percent greater GWC than punting, it means if the situation occurred 100 times, choosing the run would produce two more victories than punting.
The worst single decisions generally cost a team 10 percent to 12 percent GWC.
Because Zeus accounts for differences in the strength between the teams, an identical situation and decision will have a higher GWC for the stronger team.
Zeus can also assess the GWC impact of a player, but its primary focus is on fourth down and other so-called "binary" decisions -- extra point or two-point conversion, penalty acceptance, onside kick or long kick.
Here are some examples of fourth-down calls from the Colts' victory over Atlanta on Thanksgiving night:
Situation: Atlanta faces fourth-and-1 at the Colts 43. Score tied 0-0, 11:28 left in first quarter.
Zeus said: 19.1 percent GWC to go for it; 18.0 percent GWC to punt.
Actual call: Went for it (3-yard run for a first down).
Situation: Atlanta faces fourth-and-1 at the Colts 31. Score tied 0-0, 9:06 left in the first quarter.
Zeus said: 21.3 percent GWC to go for it; 19.6 percent GWC to kick a field goal.
Actual call: Went for it (1-yard run for first down).
Situation: Atlanta faces fourth-and-7 at the Colts 16. Score tied 0-0, 7:03 left in first quarter.
Zeus said: 22.8 percent GWC to go for it; 22.8 percent GWC to kick a field goal.
Actual call: Field goal attempt was good.
Situation: Colts face fourth-and-4 at the Falcons 47. Colts trail 10-0, 1:53 left in first quarter.
Zeus said: 57.3 percent GWC to go for it; 55.4 percent GWC to punt.
Actual call: Punt.
Situation: Atlanta faces fourth-and-4 at the Colts 12. Atlanta leads 10-7, 11:38 left in second quarter.
Zeus said: 36.2 percent GWC to go for it; 34.5 percent GWC to kick field goal.
Actual call: Field goal attempt good.
Situation: Colts face fourth-and-1 at the Falcons 49. Colts trail 13-7, 9:29 left in second quarter.
Zeus said: 66.6 percent GWC to go for it; 62.5 percent GWC to punt.
Actual call: Punt, but running-into-the-kicker penalty allows Colts to keep the ball.
Why so aggressive?
The developers of Zeus are used to sneers at some of the computer's decisions. Bower explained:
"Zeus will recommend going for it on fourth-and-short more often than NFL decision makers.
"Indy's powerful offense is more likely to pick up a first down on fourth-and-short (or even fourth-and-medium) than the typical team. Indy's play-calling consistently misses this fact.
"Atlanta is a heavy underdog and scoring points three at a time is not going to be a big plus against Indy's strong offense. They must take maximal advantage every time they are close to the goal line."
What will Zeus have to say about Sunday's Colts-Jacksonville game? Check out IndyStar.com for Zeus' decisions and commentary from Chuck Bower, one of the developers and an IU astrophysicist. He will be posting at halftime and after the game.