You’ve seen it repeatedly in the help-wanted ads: “experience preferred”; “three to five years experience in the industry required,” and the like. This is also true in baseball, where in 1996 a newly appointed Joe Torre groused openly about having to start the season with a fresh-faced, unbloodied kid named Derek Jeter at shortstop after the incumbent, Tony Fernandez, got hurt.

Had Fernandez been healthy, Jeter would be known today as one of the best shortstops in history — the history of Columbus, Ohio, that is. Lefty Sean Henn, 25, doesn’t have the kind of potential Jeter had at the dawn of his career, but he’s also being cast as runner-up in a battle to make a spot on the Yankees’ squad. His enemy: Thirty-seven-year-old Ron Villone’s long but spotty 543-game career.

Experience is supposed to provide certainty. If a player has done it before, it is reasoned that he should be able to do it again. The desire for experience expresses itself most often in the construction of major league bullpens, where relievers with a track record are valued far above the multitudes of minor leaguers who apply for major league jobs each year. This is where Henn, who was roughed up in Sunday’s exhibition game, will have a tough time beating out Villone no matter how well he pitches.

The desire to acquire experienced players is not misplaced when it comes to hitters. Hitters are a lot more predictable than pitchers, and once they establish their level in the majors, it’s a good bet that they’ll approximate that performance in seasons to come, until age or injury takes its toll. The vast majority of pitchers lack that same predictability. Pitching is such a complex art that the smallest possible disruption, be it caused by arm or mechanical problems, can turn an ace reliever into a batting practice pitcher.

Relievers are the most variable players in all of baseball. In the 2006 edition of the Baseball Prospectus annual, I looked at the top 50 relievers (as ranked by Fair Run Average) in each season over a 30-year span. On average, there was a 60% turnover among the top 50 — six out of 10 pitchers dropoff the list every year. When it comes to the bullpen, experience, which normally comes at a premium, should be value-priced, sold at more than half off the sticker.

It stands to reason that if the best relievers in baseball show little consistency, than the pitchers who are something less than that — Ron Villone, comes to mind — will be even more unpredictable. Villone, who has battled control problems from the beginning of his career, is a prime example. His career ERA is 4.78. Last season it was 5.04, though he was inconsistent even within the season, posting a first-half ERA of 2.27 and a second-half ERA of 8.35. In 2005, his season followed a similar pattern. He began the year with the Mariners, putting up a 2.45 ERA, but after a trade to the Marlins, he was battered for a 6.85 ERA over his final 23.2 innings.

The 60% rule, and Villone’s lack of reliability, should not be taken as endorsements of Sean Henn’s candidacy. Henn is an undistinguished pitcher who is being given this chance solely because he’s left-handed. Back in 2001, Henn was considered a sleeper prospect with a live arm, but subsequent physical problems sapped some of his stuff. Since then results have varied. He has looked timid in his seven major league appearances, and his record at Triple-A Columbus last season, with a 4.01 ERA and a 33:20 strikeout-walk ratio in 42.2 innings, is nothing to brag about. At the same time, time and time again we have seen pitchers like Henn, when handled adroitly by the right manager, bloom into useful major league middle-men.

Villone’s highly mixed experience, represents something like the mediocre known. Perhaps Henn’s lack of experience represents nothing more than the mediocre unknown, but he has a better chance of pushing that ceiling higher than does his middleaged (in baseball terms) rival. Fortunately for the Yankees, despite the importance generally accorded the lefty specialist role in the bullpen, it is relatively unimportant, and they can afford to experiment. If the innings that would have gone to Henn or Villone end up in the hands of one of their more competent righties, that’s not a bad thing.

As for the rest of baseball, what the 60% rule means is that many relievers who were solid refuges for their managers will provide no protection this year. Who will drop out? Will it be Cla Meredith? Dennys Reyes? Chad Bradford? Or will it, perhaps, even be the great Mariano Rivera? Putting together a bullpen is the hardest trick in baseball. The meaninglessness of experience is the reason.

Mr. Goldman writes the Pinstriped Bible for yesnetwork.com and is the author of “Forging Genius,” a biography of Casey Stengel.