Much is always said about teams finishing strong in the regular season because people believe the momentum of a superb finish generally carries on in October.

So is this belief true, or a media fabrication?

Let’s look at the numbers over the last six years:

The biggest takeaway is that over the last six years, five World Series champions have had September winning percentages greater than 0.640. The only champion not to have a winning percentage that high were the 2007 Red Sox, who won 59.3% of their games that September, which is still remarkable.

In short, teams that won the World Series have, in fact, played well in September.

That being said, just because a team finishes strong in September, it doesn’t necessarily mean they’re automatically winning the Series. After all, only one can team win the whole thing. There have been five playoff teams over the last six years who finished with a winning percentage above .700 in September and although four of them made it to their respective league championship series, only one of those teams (2011 Rangers) marched on to the World Series.

One argument against this notion is that there are an extremely limited amount of teams that can make it to the championship round. Even if everyone finished above .700 during September in a given year only two teams can eventually make it to the World Series. This is why we can’t really make much of the low correlation coefficient of 0.35.

On the flip side of the argument – of the 11 teams who finished .500 or worse in September, only two of them reached the LCS. This means that there is likely something to be said for momentum and that strong September finishes can matter.

In short, if you finish strong in September, there’s a good chance that success is going to transfer to October. It’s still better than finishing poorly, though, because the lack of momentum heading into October does seem to hurt those teams.

So what’s in store for 2013 and especially the Red Sox?

Here are the September records for all contending teams:

There’s still about half a month left, but the Red Sox are in good shape so far. You want a team to be confident and dominant heading into the games that really matter. The Sox, Athletics, and Cardinals have to be feeling good about themselves.

Meanwhile, the Rangers, Braves, and Tigers need to right the ship. There’s still plenty of time to do it, but the numbers show that over the last six years, if a team is not .500 in September, they’re not going to hang around much in October.