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By the numbers: 49ers pass defense through 9 games

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Back in September, pass defense was near the top of a near-endless list of question marks surrounding the San Francisco 49ers as they prepared to move on from the Jim Harbaugh era. Myriad losses to the coaching staff, pass rush, and secondary meant there were few familiar parts leftover from a pass defense that had finished in the top 10 during each of the preceding four campaigns.

With nine games banked, we're now beyond the midpoint of the 2015 season and are starting to get a better idea of what we can expect from the 49ers' numerous young defenders. Over the second half of the season, we're going to focus in on many of those players, providing a progress report of sorts to see where they're at in their development.

To kick things off, however, I wanted to take a quick, more general look at how the pass defense has fared. By any reasonable measure, the results haven't been good — the 49ers currently field the second-worst pass defense in football by Football Outsiders's opponent-adjusted metrics. That shouldn't come as a surprise to anyone who has watched this team play over the past few months.

But why have they been bad? What types of coverages are they using? How often are they blitzing? What routes have given them trouble?

To find out, I went back and charted all of San Francisco's defensive pass snaps through the first nine weeks. Again, we'll be looking at these areas on an individual level over the course of the next several weeks (adding data from new games as they come). For now, here's a quick data dump of how the 49ers pass defense has performed so far.

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The grey "could not determine" area comes from a small subset of plays — usually screens, goal-line plays, or play action in which the defense bit hard on the run — in which the type of coverage being used wasn't clear after the snap.

Eric Mangini has preferred zone coverages (mostly Cover 3 and Quarters, with the occasional Tampa-2 look I wish would go away forever) over man coverages at more than a 2-to-1 ratio. Man coverage has been used more often in recent weeks, but we've still yet to see a game in which man was the dominant coverage type. It's hard to say that's been the right decision, as the 49ers are allowing nearly two yards more per play when in zone.

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The other way I categorized coverage was by the number of safeties deep after the snap. The "after the snap" part of that statement is key, as the 49ers begin most plays with two deep safeties prior to the snap before rotating into their true coverage assignments once the ball is in play.

San Francisco has become primarily a single-high safety defense under Mangini, something that has been increasingly true in recent weeks. The 49ers used some variation of Cover 3 on the wide majority of pass snaps against the Seahawks and Falcons after keeping a fairly even split between single-high and two-high safety looks in the preceding weeks.

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The use of three-man pass rushes was something I had brought up on multiple occasions early in the season when Mangini was calling them on 15-20 percent of pass snaps. As you can see from the numbers above, the 49ers have allowed over 8.0 yards per play when rushing three. It's simply a bad idea to drop eight into coverage in most situations. Your pass rush goes to hell, and most NFL quarterbacks are going to find the soft spot in your coverage when you give them all day to throw.

Thankfully, the use of three-man rushes has all but disappeared from Mangini's game plans as of late. Let's hope it stays that way.

The numbers from all three of our graphics so far suggest the 49ers should be playing more aggressively and calling more man blitzes.

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This is what it looks like when you play a bunch of soft zones and aren't very good at it. San Francisco has been destroyed in the middle of the field. Opposing passers are completing nearly 80 percent of their passes at a clip of 10.1 yards per attempt between the numbers, which includes a staggering 80.5 percent completion rate on intermediate throws (10-19 yards in the air).

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Given what we know so far — the 49ers run predominantly single-high safety, zone coverages that have been eaten alive in the middle of the field — it won't be a surprise to see what routes they've struggled with the most. Seam, dig, over, and corner routes are all big Cover-3 beaters, particularly when paired with the right complementary routes, and all but the corner route attacks the middle of the field.

One thing that might be surprising is how well the 49ers have done against vertical routes down the sideline. Numbers against your standard fade routes are shown above — and most of those numbers came against the Steelers when Mangini became infatuated bringing his safeties into the box and bailing back into Tampa 2, which left the deep sideline area wide open — but the 49ers have also done very well against wheel routes and double moves (1-of-9).