But the strength of the Seahawks is in the secondary. Safeties Earl Thomas and Kam Chancellor are Pro Bowl players having great seasons, while cornerback Richard Sherman should be a first-team All-Pro selection for the second year in a row. As a result, opponents are left having to throw short passes in hopes of moving the ball. The Seahawks are allowing 9.9 yards per completion and 4.8 yards per pass attempt: both figures lead all pass defenses.

Image The top 25 pass defenses since 1970, as measured by standard deviations above N.F.L. average using Adjusted Net Yards per Attempt as the base statistic.

There is no question that Seattle has the best pass defense in the N.F.L., but how does it compare to other teams in modern history? First, we need to combine the main pass statistics into one category. The preferred approach is the advanced statistic Adjusted Net Yards per Attempt, which is similar to team passing yards per attempt (including sack yards lost in the numerator and sacks in the denominator), but adds 20 yards for each passing touchdown and subtracts 45 yards for each interception.

The league average Adjusted Net Yards per Attempt this season is 5.97, which would also be an N.F.L. record. (The previous high was 5.93, set last season.) The Seahawks have allowed just 3.40 ANY/A, easily the best in the league (the San Francisco 49ers are second at 4.62 and the Panthers third at 4.73). But since the ANY/A league average has been rising for years, we cannot just compare the Seahawks to teams of the past. We also need to measure how far from the league average each pass defense has performed.

The simplest way to measure deviation from the average is to measure the standard deviation among all pass defenses in the N.F.L. In 2013, the standard deviation of the ANY/A ratings of the 32 teams is 0.93. As a result, Seattle’s pass defense is 2.76 standard deviations above the 2013 mean of 5.97. If the Seahawks can maintain that level of dominance, it will rank as the fourth-best season since 1970.

By this method, the top pass defense was fielded by Tampa Bay in 2002, the season the Buccaneers won the Super Bowl. That season, Tampa Bay allowed 2.34 Adjusted Net Yards per Attempt; that season, the league average was 5.35 and the standard deviation was again 0.93. As a result, the Tampa Bay pass defense was 3.22 standard deviations better than average. In the postseason, the Buccaneers allowed just three touchdowns while scoring four touchdowns on interception returns.