It was a “fact” — back in the 1950s when I became a serious fan of baseball — that the team that led its league on the 4th of July usually won the league championship. (That was in the days before divisional play made it possible for less-than-best teams to win league championships and World Series.)

How true was the truism? I consulted the Play Index at Baseball-Reference.com to find out. Here’s a season-by-season list of teams that had the best record on the 4th of July and at season’s end:

It’s obvious that the team with the best record on the 4th of July hasn’t “usually” had the best record at the end of the season — if “usually” means “almost all of the time.”   In fact, for 1901-1950, the truism was true only 64 percent of the time in the American League and 60 percent of the time in the National League. The numbers for 1901-2014: American League, 60 percent; National League, 55 percent.

There are, however, two eras in which the team with the best record on the 4th of July “usually” had the best record at season’s  end — where “usually” is defined by a statistical test.* Applying that test, I found that

• from 1901 through 1928 the best National League team on the 4th of July usually led the league at the end of the season (i.e., 75 percent of the time); and

• from 1923 through 1958 the best American League team on the 4th of July usually led the league at the end of the season (i.e., 83 percent of the time).

I was a fan of the Detroit Tigers in the 1950s, and therefore more interested in the American League than the National League. So, when I became a fan it was true (of the American League) that the best team on the 4th of July usually led the league at the end of the season.

It’s no longer true. And even if it has happened 55 to 60 percent of the time in the past 114 years, don’t bet your shirt that it will happen in a particular season.

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Related post: May the Best Team Lose

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* The event E occurs when a team has the league’s best record on the 4th of July and at the end of the season. E “usually” occurs during a defined set of years if the difference between the frequency of occurrence during that set of years is significantly different than the frequency of occurrence in other years. Significantly, in this case, means that a t-test yields a probability of less than 0.01 that the difference in frequencies occurs by chance.