The right decisions sometimes work out poorly. That does not make them wrong decisions.
Spagnuolo's decision to punt on fourth-and-1 from the St. Louis 30-yard line backfired when the Cardinals' Patrick Peterson returned the ball 80 yards for a touchdown. But a statistical analysis using win probability showed Spagnuolo made a reasonable decision.
A few considerations from Alok Pattani of ESPN's analytics team:
Punter Donnie Jones had a 44-yard net average before the punt.
Gaining a single yard on fourth-and-1 would have improved the Rams' win probability to 43 percent under the circumstances (down three points, 3:30 left in third quarter, first-and-10 for the Rams from the St. Louis 31).
Gaining zero yards on fourth-and-1 would have dropped the Rams' win probability to 25.7 percent under the circumstances (down three points, 4:00 left in the third quarter, first-and-10 for the Cardinals from the St. Louis 30).
A punt from Jones with a 44-yard net average would have left the Rams with a 36.3 percent win probability under the circumstances (down three points, 3:55 left in third quarter, first-and-10 for the Cardinals from their own 26).
Given these parameters, going for it on fourth down would have improved the Rams' win probability if Spagnuolo could have assumed at least a 61.3 percent chance of converting. That percentage would change if we assumed the Rams would gain more than 1 yard on fourth down, or if we assumed Peterson would produce a long return.
The NFL average conversion rate on fourth-and-1 is 65.6 percent this season, excluding plays run inside the opponents' 10-yard line, where there is less space to operate. The Rams have converted three times in four such plays this season. They have a 64.3 percent success rate on fourth-and-1 since Spagnuolo became head coach in 2009.
Win probability takes into account how similar plays in similar situations affected game outcomes previously.
Coaches also must take into account other factors affecting probability.
In this case, Spagnuolo knew Peterson was a bigger threat than the typical returner. He knew his offensive line was struggling and might have a tough time converting on fourth-and-1. He knew his punter had been instructed to angle his punt toward the sideline or out of bounds. He knew the opposing team's quarterback, John Skelton, was not playing very well. But he also knew the Cardinals had scored a touchdown after taking over at the St. Louis 28 on Arizona's previous possession.