Measuring NFL defenses by yards allowed will provide a general idea which ones have fared better than others.
Measuring them by points allowed can do the same.
Both measures have obvious limitations. Sometimes, yards allowed are meaningless. Points allowed fails to account for the situations defenses encounter for reasons beyond their control.
Expected points are a little tougher to understand at first, but worth the time for those serious about measuring teams for what matters:
"Based on statistical analysis of 10 years of NFL play-by-play data, ESPN has created a formula that assigns an 'expected points' (EP) value to the team with the ball at the start of each play based on the game situation. Expected points accounts for factors such as down, distance to go, field position, home-field advantage and time remaining.
"The value it puts out is on a scale from about minus-3 to seven, and it basically represents 'which team is likely to score next, and how many points?' It represents the likely points not just on the current drive but also on the next drive or any subsequent drive until the score changes or the half ends.
"A lower value indicates a more favorable situation for the defense (i.e. fourth-and-20 from your own 1-yard line could be close to minus-3 EP), and a higher value represents a more favorable situation for the offense (i.e. first-and-goal is generally worth 6 EP)."
The Chicago Bears (minus-110.9), Houston Texans (minus-92.0), Arizona Cardinals (minus-31.7), San Francisco 49ers (minus-24.4), Denver Broncos (minus-21.1) and Seattle Seahawks (minus-20.1) lead the NFL by this measure, according to ESPN Stats & Information.
That means those teams' defenses have allowed fewer points than expected given the situations they encountered.
For example, let's say one defense allows a 28-yard field goal following a 10-play, 65-yard drive. Let's say another defense allows a 28-yard field goal following a three-play, zero-yard drive. Both defenses allowed three points, but the latter defense did a much better job limiting its opponent to fewer points than expected given field position. The expected points metric recognizes this.
New Orleans (plus-96.0), Buffalo (plus-88.0) and Tennessee (plus-84.7) rank last in expected points allowed.
The chart shows NFC West defensive rankings for quite a few categories, with rankings for expected points in the bottom row.