As difficult and humbling as the task of repeatedly making sales pitches to overly confident high schoolers is, doing so is absolutely necessary. There are only so many blue-chippers, and every other coach wants them just as badly as yours does.

Rather than trying to win every high-stakes recruiting battle in sight, schools must also work unbeaten paths looking for gems that other programs missed or didn't bother chasing. As they survey the fruited plains, they run into the second major difficulty in recruiting.

How do you determine which of these standouts in small town squabbles will become Achilles if thrust onto a national stage against other players who were also local heroes?

Do you look for the physical measurables and hope to be able to coach the needed fundamentals and technique? Or do you look for the players that understand the game, then attempt to strengthen and condition them into the athletes they need to become? Either way, evaluation is a tricky business that's compounded by the scarcity of talent.

Judging based on the criteria of scarcity and evaluation, a few positions stand out as particularly challenging to recruit. Unless you're Nick Saban, it might not be terribly difficult to find a soccer player who can parlay his leg into field goal skills, but it will always be difficult to keep your lines stocked and to find the right pieces for your passing game.

5. Defensive linemen

Scarcity: Edge-rushers aren't that hard to find, but strong-side defensive ends, nose tackles, or three-tech defensive tackles? The bigger a player's body needs to be to play his position at the college level, the smaller the reservoir of available talent. Consequently, defensive linemen are one of the hardest positions for recruiters to find, let alone project.

In many schemes, defensive linemen need to possess the athleticism and awareness to read the opposing lineman's block, plus the explosive power to beat that block. The defender whose size and athleticism allow him to withstand a double team, control and exploit gaps in the run game, and collapse the pocket on pass plays is a precious jewel.

Sometimes these players are longer guys who pack muscle weight on their extensive limbs, such as LSU's Ego Ferguson or Notre Dame's Stephon Tuitt, and sometimes they're stockier kids with a lot of power and a quick step, like Pitt's Aaron Donald or Virginia Tech's Derrick Hopkins. In any case, players with both the necessary athleticism and the strength to be difference-makers aren't common.

Evaluation: A kid who's 6'2, 280 pounds and able to run down a receiver from behind will always draw in scouts, regardless of where he plays his high school football.

The trickier evaluations come when projecting where players will end up after a few years of college strength and conditioning. How will the big, college-sized kids respond when their job duties go from bullying 200-pound offensive linemen and rag-dolling running backs to fighting for their lives against 600 pounds of upperclassmen? How much weight will that particular frame pack on in college?

Will that feisty, quick, 6', 260-pound defensive tackle grow into a 6'1", 290-pound all-conference contender in the mold of Donald? No? You've just wasted a roster spot. Will the 6'6, 260-pound defensive end stay that lean and quick in college? No? Then does he have the mentality to become a defensive tackle tangling with guards, or is he set on the glory of chasing sacks?

Many teams structure their defenses to limit their dependence on dominant linemen who can plug the run on first and second down and then rush the passer on third. Some will load up on quicker linemen who they can help out with stunts, or they target tough, crafty kids and teach them to make a pile while linebackers seize the glory. Bill Snyder at Kansas State has managed to field good defensive lines by choosing already grown players from the JUCO ranks and developing overlooked or undersized high school kids.

4. Tight ends

Scarcity: The big tight end who comes on the field when the offense wants to double team the edge and run the ball isn't a difficult player to find. But if you want that player to be especially mean-spirited and to stand at 6'5, 260, don't expect him to grow on a tree. And if that big, tall tight end also needs the hands and change of direction to torture defenses, he'll be an even less common find.

The tight end who helps pancake a defensive end on first down and then beats a safety in the seam on the following play is a true rarity.

Evaluation: When a high school coach finds himself with a 6'5, 240-pound athlete and feels the team can afford to put him out at tight end rather than on the offensive or defensive lines, that still hardly means that the offense will use or develop the tight end to the fullest extent.

If the team has a great running game, that tight end isn't going to put a lot of examples of great routes on his highlight tape. He might not have a quarterback who can get him the ball regularly. If it's a spread passing team, that tight end might get flexed wide to block for screens and run routes, but get less action trying to hook an outside linebacker in the run game.

Great tight ends are almost entirely projected and developed at the college level. Texas fans need both hands and a few toes to count the number of big wide receivers who were expected to move inside to tight end and didn't pan out, or bigger blockers that didn't have the hands and running ability to impact the passing game.

Stanford has had great success by taking tall kids with experience receiving and molding them into blockers through their hard-nosed culture. But even Stanford didn't have a tight end amongst its top receivers in 2013.

3. Cornerbacks

Scarcity: Every team needs more than two starter-quality cornerbacks these days, but that doesn't mean that the ideal cornerback is a common type of human. Despite his smaller size, the elite cornerback often has traits that don't show up in the genetic lottery in combination very often.

Corners who play tight coverage have to respond to cuts and movements with a major time deficit. The receiver already knows where he wants to go on the field, but the cornerback isn't totally sure until it happens. Consequently, the cornerback has to have rapid acceleration in order to close distances created by cuts and moves.

That kind of acceleration is generally found in shorter players, with a few exceptions that prove the rule, because long legs spend more time in the air and aren't as suited to changing direction. However, when the ball arrives, that smaller cornerback has to be able to compete in the air with the receiver, who might be 6'5 like Mike Evans. That means longer arms, great anticipation, and a short memory are all prized traits. For press coverage, long arms are also a must to jam and re-route receivers.

Evaluation: Many of the best corner prospects don't face quarterbacks who have the arm and freedom to attempt to beat coverage downfield until college, if even then. High school coaches facing great coverage corners are generally disinterested in having a young quarterback throw at them. Why have an 11th-grade quarterback throw to a receiver -- who might not be a disciplined route-runner -- being covered by the blue-chip guy all the college coaches are here to see? Why not pick on that shorter, slower kid on the other side?

While seven-on-seven leagues and summer camps help, recruiters don't often get a chance to see elite cornerback instincts and awareness on display in a game situation.

And great cornerbacks don't always play as much cornerback in high school. When a high school has a 5'11 kid with exceptional change of direction and blazing speed, that kid is often going to be handed the ball on offense and asked to score points. How does he do with his footwork, his hand placement, his awareness? Can he turn his hips? Who knows? But he just put a nice move on that linebacker.

Michigan State dominated opponents in 2013 playing press coverage with a pair of cornerbacks rated as two-star prospects. One of them, Darqueze Dennard, is on track to be drafted in the first round in 2014. Finding these players is very much an inexact science.

2. Quarterbacks

Scarcity: There were 125 FBS teams in 2013, but there was only one Jameis Winston. And there weren't enough Braxton Millers to fit on the teams who couldn't find a Winston.

Most college offenses are designed to be less-dependent on having transcendent passers than NFL offenses are, but every team is still hoping to land one. That makes competition for the promising ones exceptionally fierce.

Scouts frequently get caught up in whether a quarterback has the arm strength and mechanics to throw certain routes and whether he has the athleticism to make plays off-schedule. These are some of the few essential traits that are easy enough to spot from film or camps.

But the ability to lead a team? Make audibles and calls at the line? Process information after the snap? Put time into film study and understand how to evaluate opponents for weaknesses? See the field clearly while feeling pressure in the pocket? Stay calm under tremendous pressure? These traits are more rare. They don't automatically turn up in the bodies of players who have all the desired physical traits.

Evaluation: Most quarterbacks are products of their systems, and projecting high schoolers into those systems can be a challenge. What if you find a prospect with a great arm, but his high school runs the flexbone offense? What if his high school team has future scholarship quarterbacks ahead of him and he doesn't play until he's a senior?

Few high school offenses ask their quarterbacks to make the level of reads and throws that are required in college systems. The intangible traits of a great young quarterback are even harder to discern.

The pressure on recruiters to get their quarterback choices correct can't make the process any easier. Quarterbacks are often the first big stars to commit and thus lead their recruiting classes, so recruiters are snapping them up before their senior seasons. You're choosing the leader of your team of 18- to 22-year-olds based on his performance as a 16-year-old.

We saw this play out in June during this cycle, when schools further down the food chain were forced to wait on the five-stars to make their decisions. Since a school won't typically take more than one quarterback per class, most teams have to both hurry and wait.

Once on campus, quarterbacks that don't get chances to play early often jump ship, as did 13 of the top 18 prospects from the 2010 class. That puts greater pressure on coaches to get the takes right, lest they end up having to start a player who shouldn't have been on scholarship.

Teams that have success recruiting and developing quarterbacks usually choose players based on the needs of a strongly developed system, sometimes tinkering for players with particular exceptional abilities. Teams that attempt to build around future pro quarterbacks, like USC traditionally does, are at huge risk of boom-or-bust cycles if they miss evaluations or have to start underclassmen.

1. Offensive linemen

Scarcity: Every offense is designed to involve at least five players from one of the world's smallest gene pools: the tall, heavy, and quick-footed.

Defensive linemen are often more athletic and sometimes similarly sized, but their responsibilities on the football field are considerably different. An offensive lineman has to have the agility to get in front of blitzing linebackers along with the powerful base necessary to stand his ground when facing a defensive tackle with an explosive first step and 310-pound frame.

Every offense is designed to involve at least five players from one of the world's smallest gene pools.

Work on the offensive line is generally very technical and involved, requiring the ability to learn and master skills. While defensive linemen needn't necessarily be the brightest players on the field to understand how they fit into a scheme, offensive linemen have to work carefully in tandem with great awareness of their pre-snap plan along with how that will change when the bullets start flying.

You also need a portion of nastiness. When the action gets heavy, it's helpful if the offensive linemen are scrappers who like to intimidate the smaller defenders paired across from them.

The huge, powerful, tall, quick, crafty, and cruel athlete is not one of the more common varieties. And he'll probably need long arms and a wide butt, too.

Evaluation: Most offensive linemen are not 6'5, 300 pounds coming out of high school. Among 2014's 25 blue-chip tackles on the 247Sports Composite, only 10 are already listed at 300, and lesser prospects are likely to be smaller.

That means projecting the future frames of 16- to 18-year-olds and those players' willingness to pack on the needed weight and power (if that sounds easy, consider that 2013 No. 1 NFL Draft pick Eric Fisher gained 70 pounds in college after weighing 250 as a two-star prospect). And that takes time. The instances of freshmen starting on offensive lines are particularly rare, as a good line is usually built with upperclassmen. Coaches hope most linemen will emerge ready to contribute after two or three years in the program.

Like on the defensive line, it's not uncommon to find players with necessary size who were happy to bully smaller players in high school, but less wiling to embrace the massive amount of discipline, energy commitment, and threshold for pain necessary to find success. Much like Andrew Bynum on the hardwood, some kids play football in high school because they are 6'4, 280 pounds and it comes easily, not because they particularly love the game. Coaches have to be able to tell the difference.

Other kids struggle to keep on the needed weight to withstand power rush moves or drive back determined defenders. In addition to training, class, film, and games, linemen have to eat about 5,000 calories per day. If they suffer injuries, it can be hard to keep the needed weight on while rehabbing. Injuries are not at all uncommon for offensive linemen who have to navigate both the chaos in the trenches along with the risks that come from their own girth. It's simply very hard to be both a healthy person and a 300-pounder.

Evaluating kids for injury risk, the willingness to face hard coaching and challenging scenarios, and the ability to master the techniques and assignments on the line with rebuilt bodies is one of the more challenging tasks in recruiting. Given how few of these massive kids with truly exceptional athleticism exist, offensive line is the most challenging position to recruit.

As you watch and hope for your team to snag that prize, five-star running back, keep in mind the real prizes in a recruiting class. The ones that don't come so easily.

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