Bill Belichick's defensive philosophy has long been known as a very flexible one that changes every week depending on the opponents' strengths and weaknesses. Likewise he and his coaching staff are known for their aptitude at making halftime adjustments to react to the fluid game situations during a contest. A look at the 2016 New England Patriots' defensive statistics split by halves reveals just how much this philosophy is translating onto the field:
The defense that's taking the field to open a game is showing the look of one of the best run defenses in the league. Over a full game the 3.3 yards per carry would rank third in the NFL. Meanwhile the passing defense is severely below average. A completion percentage of 66.9% would be in the bottom three of the league while the 7.52 yards per pass attempt would rank 21st.
This picture changes drastically in the second half with the roles flipped around. Helped by leading at halftime in every game other than the week four matchup against the Buffalo Bills the defense is switching the focus towards shutting down the pass at the expense of the run. The 4.4 yards per carry in the second half would rank 23rd over a full game while the 6.30 yards per pass attempt would be good for fifth.
This significant deviation between the halves in the passing game gets even more extreme, however, if the week two matchup against the Miami Dolphins, when the Patriots defense played a very good first half followed by a disappearance act in the second, is removed:
Opponents completion percentage and yards per attempt in the first half rises to 68.9% and 7.73 respectively. Meanwhile the second half equivalents drop to 58.1% and 5.48 yards per attempt. To illustrate how good that 5.48 yards per attempt is: such a value over a full game would lead the league.
Of course, definite observations can't be made without comparing the Patriots' splits to the ones of the competition. Specifically compared to the best defenses in the league with the Vikings, Broncos and Seahawks.
This table is comparing the teams' passing defense ordered by net yards per attempt which factors in yardage lost to sacks. While there is a certain measure of variation in all the numbers the Patriots' variation is by far the biggest with 1.1 net yards per attempt including Miami and a staggering 2.1 without. If not for the defensive collapse against the Dolphins the Patriots' 4.9 net yards per attempt would be second only to the Broncos for the second half. Which is quite remarkable considering that unlike the three competitors the Patriots defense is not getting a lift from the defensive line in form of sacks. In fact, the raw 5.5 yards per pass attempt would top the chart.
For the rushing defense a similar picture arises with the difference between the Patriots' halves again being the biggest of the four teams in comparison.
The core principle of a Belichick defense, no matter which game plan, though is the prevention of scores and this principle is also showing in the halftime splits. Of the four teams in comparison the difference between the points per drive of 0.37 only trails the Vikings' remarkable stability at 0.11. If the Miami game is taken out of consideration this number further drops to 0.20. The Seahawks and Broncos meanwhile experience a significantly higher fluctuation at 0.66 and 0.90 respectively.