Three Questions on Probability and the Playoffs

Cards vs. Red Sox
With the All-Star game now deciding home field advantage in the World Series Cardinal fans particularly are a disgruntled lot. Why not award home field to the team with the best season record? With the advent of interleague play I think that’s a viable solution. If the leagues were still totally separate and if players didn't’t move freely between leagues, the records of the two teams really would have no basis for comparison. Since they do share at least common opponents I say base it on record.

Note: For those who don't remember the genesis for the current system was the Selig driven abomination that was the 2002 All-Star game in Milwaukee where the appropriate solution would simply have been to force the American League to forfeit. There are no ties in baseball.

But the question is, how big an advantage does home cooking give the Red Sox?

Using my Series Simulator I ran 100,000 series with both the Cardinals and Red Sox with home field advantage. With the Red Sox having the advantage the Cardinals won 56% of the time largely on the strength of their major league best .642 winning percentage on the road. When Cardinals had the advantage they won 60% of the time.

Home Field Advantage Generally
But given average major league teams what is the advantage?

The average home field winning percentage in baseball is around .540. So, given two teams that both play .520 ball at home and .480 ball on the road, the winning percentage when matched up is .540 for the home team given the Log5 formula. Running a simulation for a seven game series indicates that the team with home field advantage wins just over 51% of the time. The same holds true in a 5 game series. So given two average teams the home field advantage doesn't seem to be that significant.

Wild Card Team's Chances
Another question that comes up in relation to playoff series is how often inferior teams like a Wild Card teams would beat the best team in the league. To see how often this is the case I simulated that my Wild Card team played .540 ball at home and .490 on the road (83 wins) while my "best" team or division winner played .650 at home and .550 on the road (97 wins). With that configuration the Wild Card team still won 31% of the time in a five game series.

The lesson is that even through baseball is the "game of the long season" as George Will says, the best teams still have a considerable chance of getting beat in any playoff series just as they do in a single game. I think this point should take the edge off the criticism the Braves have taken for only winning one World Series while winning 13 consecutive division titles and especially on the A's for supposedly not being able to win while employing a "Moneyball" approach. Overall, the addition of the Wild Card adds excitement to the end of the season for many teams (Cubs, Padres, Astros, Rangers, Red Sox, and A's this season) at the cost of correlating the winner of the World Series more closely with the team who garnered the best season record. Take your pick.