With the Vikings two days from opening training camp, here's a look at two areas that helped the defense become much stingier in 2014 under the leadership of Head Coach Mike Zimmer and Defensive Coordinator George Edwards and two areas where the team can further improve in 2015.

Minnesota's defense allowed 7.1 points per game fewer than it did in 2013, which was the largest reduction of any team in the NFL. Taking away a touchdown a game amounted to better performance in multiple areas. Here are two and reasons why the trends can continue or become even better:

1) Passing yards allowed/game: The Vikings went from 287.2 in 2013 (31st in the NFL) to 223.3 in 2014 (seventh in the league).

Secondary credit: Members of the Vikings secondary, particularly Xavier Rhodes developed rapidly under Zimmer, described by several, including 2015 first-round pick Trae Waynes as a "DB guru." Rhodes led the Vikings with 18 pass breakups and was joined by veterans Captain Munnerlyn and Josh Robinson in adjusting to the new system. Safety Harrison Smith showcased his versatility with team-highs of 120 tackles (coaches' tally), 3.0 sacks and five interceptions. Robert Blanton started 13 games at the other safety spot and added 111 tackles and an interception.

This year, the Vikings drafted Waynes 11th overall after signing veteran Terence Newman, who has six previous years of experience in Zimmer's system. Newman had a tremendous spring of teaching others and competing with the first-team.

2) Sacks per pass play: The Vikings went from 6.33 sacks per pass attempt two seasons ago to 7.68 last year, moving up 16 spots to eighth in the NFL rankings by taking down quarterbacks at a more frequent rate.

Heat will stay on: The emergence of Everson Griffen, who led the Vikings with 12 sacks, 21 tackles for loss and 70 quarterback hurries, in his first season as a starter was a major reason for the improvement. The Vikings also rushed passers from more directions, capitalizing on the interior rush abilities of defensive tackles Tom Johnson and Sharrif Floyd, as well as Smith and linebacker Anthony Barr on blitzes. Shoring up the secondary allows Zimmer and Edwards to be more aggressive and exotic with the front seven defenders.

There are, of course, a few areas in which the Vikings can still improve. Here are two:

1) Rush yards allowed/game: One of the few statistics in which the Vikings took a step back will be a point of emphasis this camp. Minnesota slipped nine spots in the NFL rankings, from 16th (110.4 rushing yards/game) to 25th (121.4 rushing yards/game) year to year.

Griffen and Brian Robison play the run well on the edge, and Joseph and Floyd will be more experienced with the technical philosophy that's asked of interior defensive linemen in this scheme. Zimmer made it a point to mention that Joseph had a great spring, even though much of his work in the middle of the action is hard to spot. The defensive line's refinement against the run could help linebackers like Barr, Chad Greenway, Audie Cole, Eric Kendricks and Gerald Hodges free to make tackles earlier in run plays.

2) Opponent passer rating: The Vikings slightly improved in this stat category, going from 98.6 to 92.8, but should aim to keep reducing that success in a division that features potent passers and receiving tandems. One reason for the improvement from 30th to 23rd was trimming a TD:INT ratio of more than 3:1 (37 TD passes to 12 interceptions) in 2013 to 2:1 (26 to 13) in 2014. The Vikings can try to be more opportunistic on the back end if the front of the defense succeeds in creating more pressure. A sign of this focus is if you see a **defender doing push-ups** after a play in practice, it's likely because he dropped a pass he should have intercepted.