NFL coaches commonly call a time out when the game is on the line and the opponents are setting up a last-minute field goal attempt. They are "icing the kicker," attempting to disrupt his preparation prior to the crucial play. But does it work? There are reasons to be skeptical. Players know that such a time out is highly probable, and should thus know what's coming. The time out doesn't fool anybody, and kickers are highly specialized professionals that learn how to make plays in the face of fierce opposition. A time out is not very fierce opposition.

On the other hand, kickers are creatures of habit. They fine tune a routine and presumably employ that routine for the typical field goal; extra time takes them out of this routine, even if they know what's in store. My personal view (words to suggest that this next idea may be really screwed up) is that kicking a football is similar to hitting a golf ball. The less you think about either, the more likely you are to take a swinging leg or club and actually make solid contact with an oddly placed and perhaps oddly shaped object on the ground.

什么数据说呢？下面是最近的分析结冰的斯科特·贝瑞和克雷格·伍德的科学新闻汇总Chance.

Berry and Wood analyzed data about field goal attempts during the 2002 and 2003 NFL seasons (including playoffs). They recorded the kicker, the length of the kick, the score of the game, the time left in the game, and whether a timeout was called by the defense before the kick. They even noted whether the field was grass or artificial turf and the weather conditions (sun, clouds, rain, snow, average wind speed, temperature--—unless the games were indoors).

In these two seasons, there were 52 different field goal kickers, combining for a total of 2,003 attempts. Of these kicks, 1,565 (78.1 percent) were successful.

Berry和伍德然后看着他们定义了什么是kicks--那些用3分钟，或在游戏中（或者加时赛）更少，并会产生提前或为球队试图踢平局发生的“压力”。

There were 139 such pressure kicks, and 101 (73 percent) were successful. The defense called a timeout 38 times before the pressure kick, and 24 (63 percent) of these kicks succeeded.

Berry and Wood also estimate a probability model which allows for the influence of factors like weather to be captured. The results appear sensible, and the "icing effect" remains.

一脚做室内更容易获得成功。云也有几脚小的有益作用。雨或雪，在另一方面，减少成功的机会。大风也减少成功的概率，但不亚于雨或雪没有。

In pressure situations, the odds of success change very little (a mean decrease of 1.8 percent). However, icing the kicker in such a situation has a pretty strong negative effect.

Using their model, Berry and Wood calculate that, for an average kicker, the estimated probability of a successful 40-yard kick in sunny weather is 0.759. The estimated probability under the same conditions for an average kicker who has been iced is 0.659. "Reducing the probability of a successful kick from 0.759 to 0.659 is a very important difference," Berry and Wood report.

I concur on the magnitude - it's very large, and suggests that icing is a critical strategy. There aren't many "free calls" that a coach can make that increase the probability of winning a game by as much as 0.1.

I have two quick observations. I've often wondered whether a mixed strategy is the appropriate approach here - i.e. call a time out with probability of, say 0.65. This should put the thought in the kickers mind that he might or might not have to execute, rather than the near certainty that he will be iced. But 0.1 is perhaps too large to give up in return for the benefits of additional uncertainty. If there are diminishing returns to icing, then perhaps the spot to play a mixed strategy would be when you have two or three time outs left. Finally, the sample size is small for my tastes - the effect is identified from just 139 "pressure" kicks, and with 30+ idiosyncratic kickers in the league, 139 seems a tad small. I'd like to see the sample expanded to a decade's worth of games.

Thanks to Blake Linney for the link.