how important is opening day?

Opening day...When every team in the majors has a chance of winning the World Series.

How important, a friend at work asked, is it for a team to win on opening day?

Looking back at the last five seasons, four of five World Series winners had won on opening day. The one exception, the 2012 Giants, lost to Arizona on opening day. For the rest of this post I will assume that that statistic accurately represents the underlying dynamics of the game, and that nothing has changed in those dynamics.

There are thirty teams, and fifteen opening days. Over the past five years there have been 75 winners of opening day games, of which four won the World Series. So winning on opening day means that the probability of winning the World Series is 4/75. By contrast there were 75 losers of opening day games, of which one won the World Series. So losing on opening day means that the probability of winning the World Series is 1/75. Based on that, winning on opening day indicates that your chances are four times as great as losing does. Which, I guess, makes sense based on that four-out-of-five statistic.

Of course, the shortcoming of this analysis (aside from the broad assumption stated in the third paragraph above), is that it reads as if I'm saying that winning is what causes your odds to increase -- an example of the post hoc ergo propter hoc fallacy. I think a better guess is that winning on opening day is correlated with winning the World Series because both feats are more likely to be achieved by good teams than by bad. Which is not to say that winning on opening day has no positive effect on a team's chances of winning the World Series. All else equal, a team is better off going into game 2 with a 1-0 record than with an 0-1 record. But only slightly.

Now, do you really want to know what game is important? If you want to win the World Series, make sure you win the last game of the postseason.

If anyone has any thoughts on deepening this analysis, I'm all ears...