In trying to get a bead on tomorrow's matchup against the Chicago Bears, you might stumble across the fact that Chicago's defense is giving up 358.2 yards per game, ranking them 20th in the league in that particular stat. Which might encourage one to anticipate an easy game tomorrow.
This is why simple cumulative stats like YPG are rather empty. They never tell the whole story. You might say, "Well, surely it at least hints at the reality, right?" No, in this case it pretty much flies in the face of it. And not just because of our lingering awareness that the Bears have talent on their defense.
Did you know that the Bears are the most passed-against defense in the league? They've faced 529 attempts, 40.7 per game. In contrast, only four teams have been rushed on less (303 attempts, 23.3 per game). By nature, pass-heavy offenses average more yardage than rush-heavy offenses, so right away I'm thinking "inflated passing totals".
The reasons vary. This season, Chicago has faced Matt Ryan, Drew Brees, Aaron Rodgers, Cam Newton, Matt Stafford twice, Josh Freeman, Michael Vick, Philip Rivers, and Carson Palmer. Chicago's strength-of-schedule is currently tied for 7th hardest in the league. Those QB's may not all be leading impressive playoff-bound teams, but they are capable of putting up good passing totals regardless. For example, Newton, Freeman, and Palmer all posted good mileage (374, 264, and 301 yards respectively) in competitive games against the Bears.
You can also thank Chicago's own pre-Caleb Hanie scoring ability (on both sides of the ball) for this inflation, as they had Atlanta, Minnesota, Detroit, and San Diego buried and frantically passing to catch up in the second half.
And finally, the offenses of Green Bay, New Orleans, and Carolina really just kinda don't bother running much. Except in the fourth quarter. Then they run. To kill the clock and preserve their enormous lead.
Yet despite being burdened with more passing defense than any other team, the Bears have allowed only 16 touchdowns with 17 interceptions, a Yards Per Attempt (YPA) of 6.7, a 31% 1st-down percentage, 77.3 opposing QB rating, and 19.6 points per game. They're top ten in the league in all those categories.
YPG stats will tell you none of this. They don't take into account the effects of game situation (opposing teams rushing more to kill the clock or passing more to beat it) or opponent quality (beating the league's worst QB's in 2007 didn't make the Ruskell Seahawks defense good). For a more comprehensive stat, try Football Outsiders' DVOA (Defense-adjusted Value Over Average), which has Chicago as the #7 pass defense (and the #3 run defense).
So...YPG, pretty much telling you the opposite of reality. The Bears can stop the pass better than the two teams we faced last, against whom Tarvaris Jackson was able to game-manage.
Tomorrow's game should be a messy defensive slugfest along the lines of the Browns game, except interesting (and with bigger ramifications). Lynch has already broken 100 yards against an even better run defense than Chicago (that'd be the Ravens), and he has an offensive scheme that has exceeded expectations in opening lanes for him while still protecting the passing game. But factor in the mounting injuries on this O-line and the harshness of Soldier Field in December, and this remains a tough matchup.
The Seahawks' key to victory is really our secondary, who should be on the lookout for any wild passes on the part of Caleb Hanie.
A defeat at Chicago's hands all but washes away our playoff hopes. I'm not sure we'll be eliminated mathematically (head-to-head tiebreakers aren't considered in a three-way Wild Card tie), but the Seahawks' control of their own destiny will shift almost entirely to other teams' hands.