Monday, August 04, 2008

Do walk off wins create momentum?

Much has been said by players, broadcasters and fans about the concept of momentum in baseball. It is said that a big inning can change the course of a game or that a come back win can turn a season around. Momentum is difficult to measure because it can supposedly shift at any point in a game and is not necessarily a dramatic event when it happens. For example, a lead off single after a tough at bat could be a game changer.

While it is difficult to isolate all potential momentum turning events, one convenient group of events is walk off wins (any game won in the home team's last at bat). it is widely believed that a walk off win or sudden victory creates a boost for a team that carries over to the next game more often than not. I wanted to see if this was true, so I went to the retrosheet database and found all walk off wins between 1995 and 2007 (excluding 1999 as the data are not available). There were 2,489 of these sudden victories during the period which comes out to 7.5 per team for a season. My goal was to see if walk off winners had a tendency win the next game after their walk offs.

Sixteen walk offs fell out of the sample because they occurred in the final game of the season and thus were not followed by another regular season game. That gave me 2,473 games with which to work. I discovered that teams won 52.7% of the games immediately following walk off wins. That's more than 50% so at first glance it seems that there is a bit of a tendency for teams to win games following walk offs.

Before jumping to conclusions though, there are a couple of important factors to consider. First, walk off wins only happen at home so chances are (85% of the time to be exact) that the next game would also be at home. Since teams win 53.9% of their home games, you would expect them to have a good winning percentage in games after walk offs even without a carryover effect. Also, teams with good winning percentages tend to have more walk off wins. For both of these reasons, one would expect a win the next game after a walk off more than 50% of the time even if walk off wins had no influence on future games.

I calculated the expected winning percentage in games after sudden victories as follows: For each walk off, I calculated the winning percentage of the specific team for that year and site. For example, the 2007 Tigers won 55.6% of their home games. Thus, they would have a probability of .556 of winning a home game the day after a walk off assuming no carryover effect. I did this for each of the 2,473 walk offs and then calculated the average probability to be .528. This means, that assuming no carry over effect, we would expect 52.8% of the games after walk offs to have resulted in wins.

Since the expected winning percentage (52.8%) for games after walk offs was almost exactly the same as the actual winning percentage (52.7%), I can conclude that, in general, a walk off win has no affect over the result of the following game. As cautious as I am about accepting intangibles, this result is still surprising to me. I was not expecting a dramatic effect but I thought that sudden victories would have a small influence over ensuing games. Based on this analysis though, it appears that a walk off win is just another win.

Tomorrow, I will look at walk off losses.

The information used here was obtained free of charge from and is copyrighted by
Retrosheet. Interested parties may contact Retrosheet at ""



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