Let me say it up front. I am a Ravens fan. I was born and raised in Baltimore and spent many Sundays at Memorial Stadium watching the Colts play. I was at the complex "that night" in March of 1984. Even though I moved away from Baltimore before the "birth" of the Ravens, I have had Ravens season tickets since the beginning.

	Ray Lewis was the leader of Baltimore's punishing defense.

However, when it comes to evaluating the claim as to whether or not the 2000 Ravens have the best defense of all time, I'm going to let the numbers do the talking. And some of the numbers are not going to be too familiar to many of you.

The statistical concept of standard deviation measures how much a team dominated its contemporaries with little or no temporal bias. In the book Baseball Dynasties, we found that the best baseball teams of 1995 are as many standard deviations from the league average in runs scored and runs allowed as the best teams of 1905 or 1955. The same is true in football.

Before the Ravens won the Super Bowl, much was said that their defense could not be considered as being among the best in history unless they won the Super Bowl. So, we looked at Super Bowl winning teams beginning in 1978, when the 16-game schedule was introduced and rules were changed to open up the passing game (in baseball, the rules have remained remarkably consistent. Football has been relatively constant since 1978). And we excluded the strike years of 1982 and '87.

That said, the two defenses that stand out are the 1985 Bears and the 2000 Ravens. Here is a chart comparing those two teams in basic categories and showing their rank among all 602 team seasons from 1978 to 2000:

'85 Bears vs. 00' Ravens
Team	Points Allowed	Yards Allowed	Yards Allowed    Per Rush	Opponents Passer Rating
1985 Bears	198, 5th	4135, 17th	3.67, 145th	51.2, 5th
2000 Ravens	165, 1st	3967, 8th	2.69, 1st	62.5, 52nd

The Ravens outrank the Bears in three of these four categories, although in points allowed and yards allowed, the teams are relatively close.

Exploring Standard Deviation

Here is a brief explanation of standard deviation and how it applies to football. Standard deviation is used to measure the dispersion, or difference, in a set of numbers. In other words, it is a measure of how much the numbers differ from each other calculated using the "average" difference of each number from the mean of the group. Here's an example that might help. Applying this concept to measuring how much a team "dominated" its contemporaries is easy. Let's look at points scored and points allowed for the 2000 Rams. The Rams scored 540 points and allowed 471. The league mean for both (a point scored for one team is a point allowed for the other) was about 330.8. The standard deviation for points scored in 2000 was 82.3, which means:(540-330.8)/82.3 = 2.54; the Rams were 2.54 standard deviations better than the league average in points scored. For their points allowed total, we do the math "backwards" because more points allowed is worse. The standard deviation for points allowed in 2000 was 73.5:(330.8-471)/73.5 = -1.91; the Rams were 1.91 standard deviations worse than the league average in points allowed

The '85 Bears and the '00 Ravens are two of just six teams that allowed fewer than 200 points in a 16-game schedule; the Ravens are one of only eight teams to allow fewer than 4,000 yards. The Bears were better at defending the pass than the Ravens while the Ravens were better against the run.

Here's where standard deviation enters the equation. Standard deviation simply measures the spread in a set of numbers. The more closely a set of numbers is bunched together, the smaller the standard deviation. As pro sports leagues have matured, the standard deviation in performance among players and teams has gotten smaller. That fact means that simply comparing performance to the league average in a given season might be misleading. But using standard deviation allows you to accurately compare teams across seasons.

Look at this first:

Standard Deviations
	Std. Dev. from League Avg.     in Points Allowed	Std. Dev. from League Avg.     in Yards Allowed
1985 Bears	+2.33, 3rd	+2.02, 15th
2000 Ravens	+2.25, 5th	+2.08, 12th

Using standard deviations, these two teams are almost identical. How identical? In the book Baseball Dynasties, we simply added the two standard deviation scores (runs scored and runs allowed) to get a team's total score. If you add the points allowed and yards allowed standard deviation scores to get an overall defense score, this is what you get (overall ranking is listed second):

Overall Defense
	Overall Defense Score, Rank
1985 Bears	+4.35, 3rd
2000 Ravens	+4.34, 4th
Note: All of these scores are not rounded to two decimal places until the very end.)

It can't get much closer than that. The 1985 Bears and the 2000 Ravens are two of only three teams since 1978 to be at least two standard deviations better than the league mean in both points allowed and yards allowed in the same season. The third team? The 1986 Bears.

Based on all of this, I have to call it a draw between the 1985 Bears and the 2000 Ravens for the title of the best defense.

It's not close as to which team made me happier.

Eddie Epstein works as a consultant to major league baseball teams. He is the co-author, along with ESPN.com's Rob Neyer, of "Baseball Dynasties: The Greatest Teams of All Time." He has been a regular contributor to ESPN.com's baseball coverage.