We were looking this week for a discussion topic that would stir up both Yankees and Red Sox fans, as the two teams prepare to clash this week. We think we found one.

In this current iteration of the Yankees-Red Sox rivalry, which started when both teams made the postseason in 1995 (something that wasn't possible before the wild card was instituted), which hitters and pitchers have played the biggest roles in their teams' regular-season performance against each other?

Alex Rodriguez has hit some big home runs at Fenway Park. AP Photo/Amy Sancetta

Simply put, who has been more important: Manny Ramirez or Derek Jeter? Alex Rodriguez or David Ortiz? Pedro Martinez or Mariano Rivera?

Feel free to discuss, cajole and argue, in whatever manner you wish. But our plan is to offer a statistical take on the subject.

For that, we used a tool known as win probability added (WPA), a stat devised by sabermetric whiz Tom Tango and tracked historically via the box scores provided on Baseball-Reference.com.

WPA provides a statistical answer to the question: Whose pitching or hitting contributions were most important to their team's chance to win?

A player accumulates WPA based on situational performance. For example:

On June 3, 2007, A-Rod came to bat against Red Sox closer Jonathan Papelbon in the top of the ninth inning with two men out, no one on base, and the two teams tied.

Baseball historians have charted games to determine that in that specific instance -- tie game, top of the ninth, two outs, nobody on -- the visiting team wins 38 percent of the time.

In the confrontation between the teams' two stars, Rodriguez won. He hit a dramatic go-ahead home run.

Now, a new situation: The visiting team (Yankees) is ahead by one, with two outs and nobody on in the top of the ninth. In that instance, the visiting team wins 78 percent of the time.

So Rodriguez gets a credit of 40 percentage points (0.40) to account for his home run. Papelbon gets a debit of 40 percentage points (minus-0.40) to account for allowing the home run. The 40 comes from looking at how much the Yankees' chance to win increased (from 38 to 78 percent) after that plate appearance.

Take all of a player's credits and debits for every plate appearance in the rivalry, and you get a number that tells you how many wins that player's performance was "worth" to his team.

The beauty of WPA is that it separates the ninth-inning go-ahead hits from the home runs in 10-0 blowouts.

If you want to take the mathematics out of it, think of it this way: WPA will tell us whose hitting and pitching performances in the Yankees-Red Sox rivalry have been most meaningful.

Before we take a closer look, we should mention that we established some ground rules. This is a regular-season evaluation. The postseason is its own animal, and to compare the regular season to the postseason, or to mix the two in some manner, does a disservice to postseason play. And we're looking at only two elements of the game -- hitting and pitching. Baserunning, defense and "intangible contributions" are non-factors.

As far as the games go, everything counts. A Yankees-Red Sox game in April is as important as a game in September. Fair or unfair, our system is such that every game carries the same weight. The Yankees want to beat the Red Sox every time, and vice versa. So it all means something.