By Dan Durkin-

Editor’s note: This is the first of a two-part piece breaking down the Bears’ underachieving offense under coach Marc Trestman. The second part will be posted later this week.

(CBS) The Bears’ 2014 season can only be described as a disappointment.

Considering this failure encompasses every aspect of the football operations, from personnel evaluation to coaching to execution, there’s enough blame to blanket the area between Halas Hall and Soldier Field.

Every phase has played a part in the collective collapse, but one reality can’t be overlooked — this team was built under the assumption that the offense would score points quickly and efficiently and thus make opponents one-dimensional, allowing the newly purchased pass rush to pin its ears back and get to the passer.

Save for one Sunday afternoon in Atlanta, that plan hasn’t materialized.

Observers are fond of distilling down the struggles to a singular source, but that’s just impractical when it comes to explaining why in the second year of coach Marc Trestman’s offensive installation, the Bears offense is averaging nearly five fewer points per game this season compared to last season. The vast majority of the discussion has centered around how bad the offense has been, but not enough on the why.

In an attempt to be pragmatic in sharing my observations about what’s gone wrong, I turned to two objective sources of information — data and film.

What do the statistics say?

All numbers set forth are a year-over-year comparison of where the Bears were at this point last season compared to where they are this season.

Through 12 games, the Bears have executed four fewer offensive drives (137 against 141) than they did last season and are scoring points with less efficiency. In 2014, they’ve scored points on 39 drives (30 touchdowns, nine field goals), whereas last year, they scored points on 53 drives (32 touchdowns, 21 field goals).

Why are the Bears scoring less frequently on their drives?

Some numbers seem inconsequential. The Bears have thrown only one fewer pass than they did last year (448 against 449) and are running the ball slightly less (274 times against 298 times) and gaining fewer yards (4.1 per carry against 4.4 per carry) when they do. They’re being sacked at a higher rate (30 times against 21 times) and their yards per passing attempt average is down slightly (6.8 yards against 7.4 yards).

However, two statistics stand out — turnovers and offensive penalties.

This season, the Bears have turned the ball over 23 times (compared to 16 last season at this time), which opponents have turned into 85 points (compared to 40 last year). That averages out to giving opposing offenses two extra drives and gifting them a touchdown per game.

Finishing drives with a punt is one thing, but killing drives due to receivers and the quarterback not being on the same page, poor decision-making or shoddy ball security is losing football.

The Bears have committed 42 offensive penalties this season (compared to 28 last year at this time), which already eclipses last season’s total of 37. Their next false start penalty will double last season’s total of nine. Such numbers speak to an overall lack of discipline and focus. Furthermore, the Bears play like a fragile offense that’s unable to work through adversity and move on from mental mistakes to keep drives alive.

Surely, more consistency and resiliency should be expected from a starting offensive unit that costs $55.6 million, the second-highest price tag for an offense league-wide.

What does the personnel show on film?

To be crystal clear, there’s enough talent on the current roster to compete and succeed. However, there were some missed or over-evaluations of the assembled talent, as there are pieces missing.

Last season, the Bears benefited from stability along the offensive line more than anything else, as the same five players started all 16 games. Continuity can make up for shortcomings, as knowing the tendencies of the person next to you helps develop communication and trust, which are crucial to effective line play.

This season, the Bears have used seven different starting combinations along the offensive line, which has had an adverse effect on the offense. Right tackle was a problem area for the Bears last year, and it remains the same this season.

Jordan Mills has given up the most quarterback sacks and has the most penalties among the group. Injuries have hampered Mills this season, but even when healthy last season, he demonstrated the same issues — sloppy footwork, inconsistent hand placement, unbalanced when anchoring and an inability to mirror on the edge.

Michael Ola has displayed versatility, starting games at left guard, left tackle and right tackle, which makes him an ideal swing player. However, it’s clear the team must make a concerted effort to shore up this position in the offseason. Perhaps they should consider moving right guard Kyle Long out to the edge, as he has emerged as both the team’s best pass protector and overall lineman.

This isn’t to give left tackle Jermon Bushrod or center Roberto Garza a pass for their performances, either. Granted, the Bears frequently keep Bushrod “on an island” as they slide and create their three-man blocking surfaces to the right. The Bears paid Bushrod to win one-on-one matchups, but he’s had his own issues in both pass pro sets and run blocking.

Garza is a leader and a has a high football IQ, but it’s clear the end is near. Garza can struggle to get movement on shade techniques in the run game and doesn’t have the feet to match some of the younger defensive tackles he faces.

Consequently, the Bears are forced to use a sixth offensive lineman on about 15 percent of their snaps per game to both create gaps in the run game and provide help on the edge in the passing game. In turn, this takes an eligible receiver out of the mix.

Trestman is a protect-first play-caller who uses running back Matt Forte on check releases, which means the Bears keep six or seven in to protect against five or six rushers, leaving five or six defenders to cover (sometimes just) three eligible receivers. Those numbers clearly don’t favor the offense and allow defenses to bracket with both deep and underneath defenders.

Thus, the Bears need a true second tight end who can contribute both as an in-line or “Y” as well as a “F” or “H” back. The Bears started the season with Matthew Mulligan (who was cut) and Dante Rosario to function in these roles as fullback Tony Fiammetta (released with injury settlement) was never healthy enough to play.

The Bears’ two-back running game now has a misfit piece with Rosario acting as the lead back, and their two-tight end packages frequently feature an offensive tackle.

Finally, the Bears lack speed at the wide receiver position. Brandon Marshall and Alshon Jeffery win by out-leveraging defenders, not by out-running them.

Without a true burner in their preferred 11 (one running back, one tight end, three wide receiver) personnel grouping who can take the top off of a defense, the Bears don’t occupy the safety frequently enough for the inside-breaking routes run by No. 1 (closest to the sideline) receivers to materialize.

Granted, Marshall and Jeffery have played the majority of the season with nagging injuries, but this only makes the need for more speed at wide receiver more glaring. Because it takes them longer to get into and out of their breaks and settle into the soft spots in zone defenses, that requires the offensive line to hold their blocks a click longer and quarterback Jay Cutler to trust the protection to hold, which doesn’t always happen.

In the end, teams can effectively and comfortably drop into their split-safety coverage shells and run them tighter to the line of scrimmage, which shrinks the area that the Bears offense has to operate within.

Clearly, there’s been a variety of factors at play with the sputtering offense that span personnel, play-calling and execution.

In the next part of this series, we’ll take a closer look at what’s happening on the field using All-22 coaches film.

Dan Durkin covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @djdurkin.