Chiefs, Texans rank among most improved offenses and defenses

— -- Consistently strong play on both sides of the ball has helped the Cincinnati Bengals, Arizona Cardinals and Carolina Panthers combine for a 32-4 record through Week 13. The Seattle Seahawks and Kansas City Chiefs are taking a different path to late-season relevance, overcoming sluggish starts to climb sharply in their conference standings.

December's race to the 2015 NFL regular-season finish line will showcase teams in varying stages of performance. Some teams will sprint, gaining speed all the way. Others will limp. With that in mind, I've compiled the five offenses and five defenses that have shown the greatest statistical improvement so far this season. Some of these units belong to true contenders, such as Denver and Seattle. Others belong to teams fighting their way toward respectability.

To analyze offenses and defenses, I divided every unit's performance into two six-game blocks and then ranked each unit within each block. Expected points added (EPA) per game was my metric of choice, for reasons explained here. Once the rankings were set, I calculated how many spots each unit moved up or down from its first six games to its subsequent six games. For example, the Chiefs' offense ranked only 27th through six games, but it has ranked fifth for games seven through 12. The plus-22 change in offensive ranking represented the largest gain for any offense in the league.

The Chiefs also ranked among the top five on defense. (No wonder they've won six straight.) Which other teams have made the most significant jumps? Let's dive into the lists.

Most improved offenses

1. Kansas City Chiefs (+22)
Games 1-6 rank: 27th
Games 7-12 rank: 5th

The Chiefs were supposed to be finished once they lost running back Jamaal Charles to a season-ending knee injury in their fifth game. Instead, they have transitioned into one of the NFL's best teams. Alex Smith ranked 32nd out of 33 qualifying quarterbacks in Total QBR through six games. He trails only Carson Palmer over the past six games, tossing eight touchdown passes without an interception. Receiver Jeremy Maclin has seen his production spike of late, but the Chiefs are 2-2 when he reaches 100 yards receiving. Kansas City has the look of a team that started slowly, stayed the course under a strong head coach and improved as the season progressed.

2. Seattle Seahawks (+21)
Games 1-6 rank: 23rd
Games 7-12 rank: 2nd

There was a good chance this would happen, but logic said Marshawn Lynch and Jimmy Graham would be part of the Seahawks' usual late-season offensive resurgence. Quarterback Russell Wilson has been the driving force, in part because his offensive line is giving him a chance now that the team finally moved Patrick Lewis into the lineup at center. Wilson's abilities as a pocket passer, appreciated by some but not all, have been on display lately. He has completed 59 of 69 passes (85.5 percent) from the pocket over the past three games, with 10 touchdowns and no interceptions on those throws.

3. Denver Broncos (+19)
Games 1-6 rank: 31st
Games 7-12 rank: 12th

Having full command of an offensive system built around him helped Peyton Manning excel into his late 30s. The Broncos' decision to scrap that system precipitated Manning's demise once it became clear Denver's offensive line could not protect him or power an effective ground game. That explains why the Broncos ranked 31st in offensive EPA per game through six contests. Manning did subsequently produce at a high level for one game, against Green Bay, before his flameout performance against Kansas City cleared the way for Brock Osweiler to take over. Denver's ground attack has come to life through improved line play in conjunction with Osweiler taking snaps from behind center, where coach Gary Kubiak would prefer his quarterbacks to be from a design standpoint.

4. Tampa Bay Buccaneers (+17)
Games 1-6 rank: 25th
Games 7-12 rank: 8th

Quarterback Jameis Winston ranked 33rd in interception rate (5.3 percent) through the season's first four weeks, continuing a trend from his final college season. Since then, he ranks eighth (1.6 percent), a leading reason Tampa Bay has become so much more efficient. Running back Doug Martin and the ground game also needed a few weeks to hit high gear under new coordinator Dirk Koetter.

Tampa Bay's 45-17 victory over Philadelphia in Week 11 produced the second-best offensive EPA game in the NFL this season, at plus-28.5 EPA, behind only a plus-31.9 for New England against Jacksonville. The Buccaneers' home game against New Orleans in Week 14 provides another inviting opportunity for Tampa Bay to continue its offensive rise.

5. Chicago Bears (+13)
Games 1-6 rank: 27th
Games 7-12 rank: 14th

The Bears scored no touchdowns on 19 drives when backup Jimmy Clausen, since released, played against Arizona and Seattle early in the season. Starter Jay Cutler has been at the controls ever since. He ranks 12th in Total QBR (63.2), including second on third down. Cutler's interception rate stands at 1.9 percent, which would be a career single-season low. It was 3.2 percent last season. Improved health for receiver Alshon Jeffery is another factor. He played 71.4 percent of the snaps in the Bears' past six games, up from 29.3 percent over the first six.

Most improved defenses

1. Chicago Bears (+25)
Games 1-6 rank: 31st
Games 7-12 rank: 6th

The Bears opened the season against Green Bay and Arizona before somehow finding themselves in a 37-34 shootout defeat at then-winless Detroit. Those three games wrecked the Bears' defensive statistics through the first six games. They have drawn a series of easier assignments since then: St. Louis' league-worst offense, Denver in Osweiler's first start, Green Bay with the Packers mired in a free fall and then San Francisco last week. Chicago has probably played about as well as expected on defense when opponents are taken into account.

2. Houston Texans (+19)
Games 1-6 rank: 22nd
Games 7-12 rank: 3rd

The Texans' slow start on defense came as a surprise after they ranked second in defensive EPA last season. Some coaches in the league thought losing defensive line coach Bill Kollar to Denver in the offseason stood out as a significant subtraction. Whatever the case, this defense has largely recovered, holding Cincinnati and New Orleans to six points apiece. A rough outing at Buffalo in Week 13 stands as the fourth-worst defensive EPA game for Houston during the Bill O'Brien era. The other three also were this season, including two against teams that have subsequently fired their offensive coordinators in Miami and Indianapolis. That is puzzling.

3. Kansas City Chiefs (+18)
Games 1-6 rank: 19th
Games 7-12 rank: 1st

After being unable to beat Manning, the Chiefs could go down as the team that sent him limping into retirement. Their signature defensive performance at Denver in Week 10, complete with five interceptions, had to be cathartic. So why the big improvement? There are obvious personnel-related reasons. Cornerback Sean Smith served a three-game suspension to open the season. Safety Eric Berry was coming off cancer treatment. Linebacker Derrick Johnson and nose tackle Dontari Poe were coming off surgeries, as was lineman Mike DeVito. The defense has held up recently despite losing top pass-rusher Justin Houston to a knee injury.

4. Tampa Bay Buccaneers (+16)
Games 1-6 rank: 26th
Games 7-12 rank: 10th

There were scant signs in 2014 that coach Lovie Smith was succeeding in establishing the defensive culture that marked his tenure with Chicago previously. This Tampa Bay defense struggled in the opener against Tennessee and had rough games against Carolina, Jacksonville and Washington over the first six. The schedule has cooperated lately. Tampa Bay swept an Atlanta team with a struggling Matt Ryan behind center. There was also a victory over Matt Cassel-led Dallas and a Philadelphia team that ranks 30th in Total QBR for the season.

5. New England Patriots (+12)
Games 1-6 rank: 20th
Games 7-12 rank: 8th

Rough defensive games against Pittsburgh in Week 1 and the New York Jets in Week 7 caused the Patriots to lag in the defensive rankings early. They have held Miami, Washington and Buffalo beneath 14 points over their past six games. They also limited the Eagles' offense to two touchdowns Sunday. That's impressive, considering linebacker Jamie Collins has played only 24.6 percent of the defensive snaps over the past six games, down from 97 percent previously.

Quick final thoughts

It was possible to look at overall offensive and defensive improvement by combining EPA figures on both sides of the ball and then calculating rankings across six-game splits. The three teams that were most improved overall -- Kansas City, Chicago and Tampa Bay -- all had the types of veteran head coaches we might envision having the ability to hold things together during rough patches.

The Chiefs' combined offensive and defensive EPA ranking improved from 25th in the first six games to first over the past six. The Bears went from 32nd to 10th and the Buccaneers from 29th to eighth. The next teams were Jacksonville, which went from 30th to 16th, and Houston, which went from 22nd to 11th.

Biggest fallers on offense: Falcons (5th to 25th), Dolphins (14th to 30th), Chargers (9th to 24th), Browns (17th to 31st), Colts (15th to 27th) and Packers (4th to 15th). The Rams' offense was 32nd both times.

Biggest fallers on defense: Eagles (2nd to 31st), Titans (9th to 28th), Vikings (7th to 21st), Dolphins (14th to 25th), Redskins (13th to 23rd) and Jets (3rd to 13th).

Biggest fallers in combined offense and defense: Eagles (8th to 29th), Dolphins (14th to 30th), Rams (18th to 31st), Jets (2nd to 13th), Packers (5th to 15th) and Falcons (9th to 19th). Note that the Bills and Saints did not change at all. Buffalo was 12th both times. New Orleans was 27th.