If your assumption is that the Bills' defense has been better than the Bills' offense this season, you'd be correct, although the difference may be smaller than you'd think.
Brian Burke, who does football analytics for ESPN, posted the following graphic Tuesday that charted each team's expected points added on offense versus on defense. Offense is on the X-axis going across, and defense in on the Y-axis going up and down.
Expected points added is a useful metric that weighs the value of each play. Every situation on the field corresponds to an expected point value, given what teams have historically ended up doing from that down and distance. If a play results in a situation with a higher expected point value, the play was positive, and if it results in a lower expected point value, it was negative (and flip those around for defense). For example, a 5-yard run on first and 10 from the 20 would be more valuable than a 5-yard run on third and 10 from the same spot.
Team Expected Points Added per Game. X-axis is offense. Y-axis is defense. Up and right are good. "Fisher Line" added to demarcate net mediocrity. pic.twitter.com/JllkBC46CD
— Brian Burke (@bburkeESPN) December 5, 2017
The Bills rank surprisingly low on this chart for a team that still has playoff potential, and it's interesting that both units rank in the negative. The Bills' defense has an average EPA of roughly -1, while the defense is even worse at about -4.5.
Elsewhere on the chart, Doug Marrone's Jaguars are killing it on defense, ranking about as high as the Patriots' offense. The Eagles may be the most well-rounded team, while the worst team is clearly the Colts, who visit the Bills next week.