There has been a lot of debate as to when to draft a team defense in redraft leagues, and over the years I have seen the first one come off the board in single digit rounds while many times other owners take one with their very last pick. While it is clearly important to have at a minimum an average defense, many times the investment in a projected top defense is too costly. On many occasions, defenses get drafted similar to their rankings and production from the previous season. This got my thinking as to how NFL defenses have performed from one year to the next. I did some digging and here's what I found.

For starters, scoring for team defenses in fantasy football is SO VARIED that it would be impossible to define a standard scoring system. Reviewing the leagues that I personally play in, some give or take away points for yardage and points allowed (and some ignore either category). Many have different point values for turnovers, some count forced fumbles but not others, some count blocked kicks and special teams touchdowns, etc. The bottom line is that the scoring systems are all quite different. Because of that, the rankings for defenses in each league were somewhat varied, although rarely did a team have a radical difference in its ranking from one league to the next.

I elected to look only at NFL rankings for each franchise with regard to points allowed and yardage allowed. That's it--no turnovers, sacks, return TD, safeties scored, or anything else. In theory, if a defense minimizes the yardage and points it gives up, the odds are pretty good that it will do well fantasy wise. Getting accurate numbers for all the other categories would be a challenge, as few sites list all the information in an easily accessible format. In reviewing data in those other categories, I have concluded (without thorough review and analysis) that many times there is no rhyme or reason for which teams do well in those categories. Over the years, defensive TD, turnovers, safeties, and the like vary greatly from year to year. Sacks can often stay within a range, but for the most part many of these peripheral categories are difficult to predict.

I added each team's yearend ranking in points and yardage allowed for each of the past 10 seasons. The best a team could do in any season would be a score of 2 (ranking first in both categories). The worst a team could do in any season would be a score of 64 (ranking last in both categories). Four teams did have a year with a perfect score of 2. No team was so anemic to score a 64, but 11 teams did score 60 or higher. Again, in theory, dividing the number by 2 would give an approximate ranking for where the team might have ranked defensively in that given year.

To make things a bit easier to understand, I made a color-coded table to help people visualize how teams performed each year. Teams in the top 3 colors (red, orange, yellow) should be considered in the top half of the league (Top 15) and those in the other categories would be below average to poor.

All that being said, here's what the numbers seem to reveal.

• Tampa Bay was far and away the best and most consistent defense, scoring in the highest bracket 6 times and second bracket 4 times-the only team that did not score worse than 20 points in all 10 seasons.
• Pittsburgh also had a tendency to have a tough defense from year to year and in their off years they were still quite good.
• Surprisingly, Denver has sneakily been a very consistent defense and many times can be had much later than some of the other common picks for team defenses.

In terms of other global observations . . .

• 44% of the time teams stayed within 10 points of the total they had the season before (either higher or lower), so in theory staying within 5 rankings if we said half of the score equaled one spot in the defensive rankings.
• 26% of the time teams scored 10-19 points away from the total they had the season before (again either higher or lower) or theoretically more than 5 but less than 10 spots in the rankings
• 17% of the time teams scored 20-29 points away from the total they had the season before (again either higher or lower) or theoretically more than 10 but less than 15 spots in the rankings
• 10% of the time teams scored 30-39 points away from the total they had the season before (again either higher or lower) or theoretically more than 15 but less than 20 spots in the rankings
• 3% of the time teams scored 40+ points away from the total they had the season before (again either higher or lower) or theoretically more than 20 spots in the rankings
• The biggest one-year increase was Carolina at up 52 points (from 59 to 7), while the largest drop-off was Tennessee at 46 points (from 4 to 50).

Again assuming that 2 points = 1 spot in the defensive rankings, on average:

• 5.2 teams per year would rank +/- 10 spots in the defensive rankings
• 3.0 teams per year would rank +/- 15 spots in the defensive rankings
• 1.2 teams per year would rank +/- 20 spots in the defensive rankings
• 9.4 teams per year would rank at least +/- 10 spots in the defensive rankings (or close to 30% of all teams in the league)

Of the teams that scored in the highest category (red), 28% did as well the next year, 31% fell to the next highest tier (orange), 19% fell to the third tier (yellow), and 22% fell beyond that, so 78% stayed in the top half of defenses.
Top Tier Year X + 1: 59% Top Third/28% Middle Third/13% Bottom Third

Of the teams in the second highest category (orange), 15% moved in to the elite group (red) the next year, 31% stayed the same, 20% fell into the third tier (yellow), and 33% fell beyond that.
Second Tier Year X + 1: 47% Top Third/35% Second Third/18% Bottom Third

Of the teams in the third tier (yellow), 8% moved into the top tier (red), 12% moved into the second tier (orange), 25% stayed the same, and 54% fell into the bottom half of defenses.
Third Tier Year X + 1: 20% Top Third/56% Middle Third/24% Bottom Third

Of the teams in the fourth tier (green), 11% moved into the top tier (red), 9% moved into the second tier (orange), 20% climbed into the third tier (yellow), 20% stayed in the same tier, and 40% fell into the lower tiers.
Fourth Tier Year X + 1: 20 % Top Third/40% Middle Third/40% Bottom Third

Of the teams that scored in the fifth tier (blue), 13% moved into the top tier (red), 13% moved into the second tier (orange), 25% climbed into the third tier (yellow), and only 8% moved into the fourth tier (green). 13% stayed in the same tier while 30% fell to one of the two lower tiers.
Fifth Tier Year X + 1: 25% Top Third/32% Middle Third/43% Bottom Third

Of the teams that scored in the sixth or seventh tiers, 4% moved into the top tier (red), 4% moved into the second tier (orange), 15% climbed into the third tier (yellow), 26% moved into the fourth tier (green). A slight majority (51% ranked in the remaining lower three tiers.
Sixth Tier or Worse Year X + 1: 9% Top Third/40% Middle Third/51% Bottom Third

On a separate note, I looked into how well teams did the following year after allowing 14 or fewer NFL points per game (224 on the season), as the Bears allowed only 202 points last season. I found 17 other teams in the past 20 years where that was the case. And the huge majority of the time those teams did not fare very well in the following year.

The 17 teams averaged 204.6 points allowed in YEAR X but a somewhat alarming 308.2 points allowed in YEAR X + 1. Of the 17 teams, 10 of them allowed 100+ more the following season. Only the 1992 Saints did better the year after allowed fewer than 240 points.

If the 2006 Bears were to follow this trend exactly, then they would allow 306 points this year. By comparison, the Cowboys allowed 308 points last year and ranked 12th in points allowed. The 1986 and 1988 Bears were two of the teams of those 17 that I mentioned-and in Year X + 1 they allowed 300 and 377 points respectively.

This year, the Bears team defense currently has an ADP of DEF1, going on average as the #81 pick overall in fantasy drafts. If you believe in the numbers side of football, you may want to avoid the Bears defense this year-no matter how solid they look defensively. To be clear, I DI NOT look into how all other teams fared in Year X + 1, so I do not have a true comparison for how the elite defenses in points allowed did the following year. It is possible that other teams did equally bad or worse from year to year, but again I did not review that. My main concern was seeing how the elite teams did, not all the others.

There are some plausible explanations as to why defenses fare so differently from one year to the next. For starters, key players could be healthy or injured, so teams on the wrong side of the tracks could stay healthy and perform much better (or the big kahunas lose some key keys and struggle). Teams could also have made changes at head coach or defensive coordinator, which could give a shot in the arm after doing poorly the year before.

Another key component could very easily be luck and circumstance. Tipped passes that one-year were interceptions could suddenly fall to the ground instead. Fumbles could bounce differently. Measurements that fell short one season became first downs by an inch the next. Maybe a team's OFFENSE did better and kept the defense off the field. Or maybe the offense turned the ball over and the defense was on the field way more.

The bottom line is that there are a lot of variables that could factor into the difference in rankings. Certainly, a good scheme, great talent, and a good coaching staff and game plan SHOULD keep a defense near the top, and in some ways the higher ranked teams in this study tend to show that.

Again, while I am not suggesting to take or avoid a team defense based solely on historical data, at least it gives us something to consider when trying to figure out which defenses to take and when.

In my opinion, the results show that the majority of teams seem to bounce around in terms of how well or how poorly they do from year to year. There does appear to be a few teams that have been able to stay consistently near the top, but there aren't many of those. It still looks like waiting on a defense looks like a decent option, especially given that many times the top defenses fail to repeat from year to year.