With free agency receding in the NFL's collective rearview mirror, teams have one more chance to plug the holes in their starting lineups before training camps begin: the 2014 draft.

But wait—aren't teams supposed to plug holes in free agency and build through the draft? Not anymore. As Bleacher Report NFL Lead Writer Michael Schottey recently wrote, the old philosophy of drafting rookies and developing them has fallen by the wayside.

In today's win-now NFL, when teams need an instant upgrade, they plug in a first- or second-round rookie and consider the hole filled.

But what percentage of rookies actually step in and play at a high level? Are there steeper learning curves at different positions? What position is the hardest for rookies to step in and master? 

Draft and Develop?

Using the Pro Football Reference Player-Season Finder, we're able to bring up historical players by position and draft position and compare their statistics.

Many draftniks agree it's difficult (if not impossible) to assess a draft pick until he's played at least three years in the NFL. Using that standard as a guide, let's review the last 10 draft classes' worth of first- and second-round picks who've had at least three years to prove themselves.

Let's also eliminate the true "busts"—any player who wasn't on an active roster for at least three seasons won't be included.

In Pro Football Reference Approximate Value (AV), we have a handy one-number stat that measures a player's contribution to his team's overall output. It's not perfect, but it's a great cross-positional stat that paints a clearer picture of how much a player was able to contribute on the field in any one season.

Here's how these classes break down, in average approximate value, by position:

Across most positions, we see a familiar, sensible pattern emerge: slow rookie season, big step up in the second season, mild increase in the third. Averages for tight ends, defensive linemen, cornerbacks, wide receivers, offensive linemen and linebackers roughly follow this pattern.

However, the averages for safeties and running backs don't. They start out contributing a little more than most positions, take a big jump up in the second year and regress in the third.

Quarterbacks do their own thing, too. They start out with the third-lowest average rookie AV of all the positions—then take a massive leap, becoming the most productive position group in their second year. Then they plateau, on average, with negligible improvement in the third season.

Of course, these averages include lots of players who barely contributed at all. Can we find the players who really stood out?

After taking the annual AV averages at each of the position groups, we can find which players beat that average in their first, second or third season. These are whatever we want to call the opposite of busts: players who contribute like quality first- or second-round picks. Hits, if you will.

Let's see how often the rookies at each of the position groups "hit" across each of their first three years in the NFL:

We see a vaguely similar pattern overall, but now there are large differences in scale.

Running backs and quarterbacks rarely excel as rookies. Receivers seem to have an immediate impact more often but don't seem to collectively get better over three years. Linebackers follow the same curve as receivers, but roughly 10 percent higher, across the board.

Linemen and cornerbacks explode in their second and third seasons; safeties and running backs start hot, get hotter in year two and fall off the map. Quarterbacks, again, rarely excel in year one, but frequently do in years two and three. First- and second-round tight ends frequently dominate.

Different Paths for Different Prospects

These are interesting figures, but they're averages. Every position group had players that played well in each of their first three seasons, disappeared in each of them, started off hot but trailed off or showed a steady progression.

Let's define five different career development paths and see what proportion of each position's top rookies followed that path.

We'll have "instant impact" players. They played at an above-average level as rookies and again in at least one of the other two years. These are the players every team wants; they're both quick fixes and long-term solutions.

Then, the "one and done" players, those who managed one good season in their first or second year but didn't follow it up. These players step in, start and make plays but typically don't become long-term solutions.

The next category is "quick developers," those who didn't play like quality starters as rookies, but did in their second and third season. They may not have exactly plugged the hole they were drafted to fill in year one, but they became vital in the medium and long term.

Then we have "slow developers," those who follow a more traditional learning curve. These players only contributed one above-average season, and it was their third in the league.

Finally, we have the "washout" group; these players didn't have a single season as a decent starter or impact rotational player in any of their first three years. Some of these players eventually caught on (hello, Aaron Rodgers), but most of them didn't.

First we have quarterbacks, who are represented by a relatively low percentage of instant-impact players (24.3 percent) and a higher-than-usual washout rate. The other three groups are fairly typical in size, though quarterbacks have one of the biggest combined groups of quick and slow developers.

This dovetails nicely with our earlier graphs: If a top quarterback prospect isn't a world-beater in year one, don't worry. After three seasons, 54 percent of first- and second-round quarterbacks still look like they'll be solid going forward. If he isn't playing well after three years, though? Worry.

Running backs, as always, make up one of the grimmest categories.

A whopping 43.1 percent of running backs drafted in the first two rounds couldn't muster a quality season in any of their first three. Combined with the one-and-done group, only 43.2 percent of Round 1 and Round 2 running backs looked like winners after three years.

Receivers also have a high washout rate. However, an impressive 30.4 percent of receivers were instant-impact guys. A relatively small one-and-done group, 55 percent of first- and second-round receivers finished their first three seasons looking like solid contributors.

Interestingly, the old fantasy football notion that receivers really come to life in their third season isn't supported by this data; they had one of the smallest populations of slow developers.

Tight end just isn't a premium position. Modern pass-first offenses rarely need a "third tackle" next to the offensive line, so a two-down blocking beast who can also get open isn't something teams eagerly reach for. Instead, a committee of one-dimensional tight ends can do the trick—and they can be had much later in the draft.

So, only players with rare combinations of elite size and elite athleticism go in the first two rounds, and those players rarely miss. Even when they don't reach their potential (see Pettigrew, Brandon), teams rarely feel pressure to find a replacement.

Just 14.8 percent of top-pick tight ends go three years without standing out. An incredible 81.4 percent of them make a big impact in either their first or second years. Just one tight end in this entire data set, Anthony Fasano, flopped in his first two seasons and blossomed in his third. Generally speaking, if a tight end hasn't become good after two years, he isn't going to.

Recently I dissected the meaning of a "safe" pick. Offensive linemen are usually considered safe—and this data bears that out: 65.3 percent of offensive linemen look like winners after three seasons, thanks to a generous helping of instant-impact guys and the second-fattest slice of quick-developing prospects.

The defensive line is similar...yet a little bit different.

A few more defensive linemen succeed from day one, compared to their offensive counterparts; quite a few more wash out completely. The biggest squeeze, though, is on the slow-development group: Just 8.4 percent of defensive linemen put it all together in year three.

The old philosophy of drafting used to be, "If you don't have a quarterback, take one." The new philosophy might be, "If you need a linebacker, take one."

A league-best 37.7 percent of linebackers drafted in the first two rounds step in and excel, and maintain a high level of play after that. Overall, 60.7 percent of these prospects will pan out over the first three years—but there's a slightly higher-than-usual rate of one-and-done types, so a player who excels early may lose his edge.

Cornerback might be the position teams most excitedly overdraft on raw ability—and boy, does it show. They have a one-and-done rate of 16.7 percent, third-highest overall. 

Despite this, cornerbacks have an advantage nearly every other position group doesn't: a natural way to ease players into the position, with a myriad of defined roles and substitution packages. Their slow-developer group is by far the biggest at 21.2 percent.

A whopping 37.9 percent of cornerbacks taken in the first two rounds over this period didn't play well in their first season—but turned it on in year two, year three or both. Perhaps the willingness of NFL teams to draft cornerbacks on athletic ability and let them develop results in the lowest washout rate of any position group, just 12.1 percent.

Safeties have the weirdest development curve of all.

Look at that whopping one-and-done group! Nearly one-third of all safeties drafted in the first three rounds made an early impact and fell off the map. All told, 77.7 percent of these safeties put up at least one quality season in their first two, but only 51 percent seemed to be long-term fits after three seasons.

Hazarding a guess, teams overdraft athleticism at the safety position. They tend to "swim" as rookies, fly all over the field as sophomores and then opposing quarterbacks figure them out.

An alternative theory: Defensive coordinators gradually put more responsibility on the shoulders of safeties who show well early on, meaning their statistics plummet as they play within the defense.

Maybe it's a little of both.

Measuring the "Hit" Rate

So, let's bottom-line it. When teams draft a player in the first two rounds, they scratch that position off their shopping list.

But, how often will those players actually go out and play like quality high-end draft picks in their very first year?

Here's the breakdown:

As we saw above, safeties and linebackers are the surest bets. Players with standout physical tools at those positions step in and play well about 43 percent of the time—almost as often as not.

Cornerbacks, defensive linemen and tight ends make up the next tier, stepping in and playing well around 37 percent of the time. Offensive linemen and wide receivers are close behind, at about a one-third hit rate.

Finally, the bad news: A little over a quarter of first- and second-round running backs and quarterbacks step in as rookies and excel.

Teams entering the 2014 NFL draft without a quarterback should still take one—but they should know they'll likely play all of 2014 without a good one, anyway.