It could be an injury, or maybe it’s academic ineligibility. Sometimes it’s personal.

Maybe it’s a guy deciding not to use his fifth year of eligibility, and maybe it’s a recruit making a last-minute decision change.

Whatever the reason, attrition is bound to happen in college football.

“I don’t know any program that has no attrition from (signing day) until the day freshmen report,” Indiana coach Gerry DiNardo said.

When programs hand out their NCAA-allotted 85 scholarships every year, they do so with the knowledge that some of those scholarships will go to waste. While football teams have a maximum of 85 scholarships at their disposal, they rarely get to have 85 scholarship players in the program.

Some conferences — the Big 12, the SEC, the Pac-10 — have adopted a measure known as “oversigning” to combat the problem. By allowing teams to sign more than 85 players — with the assumption that some of them will be gone before the season starts — teams are more likely to use all of their scholarships.

Frustrated by the fact that his team only has 62 scholarship players on its roster this year, DiNardo launched a campaign to bring oversigning to the Big Ten.

“I had nine years of experience with (oversigning) in the SEC,” DiNardo said. “I don’t think there’s any way, at most universities, that you can maintain your roster at 85 scholarships if you’re not permitted to oversign.”

DiNardo won his battle. In October, the Big Ten decided to allow teams to oversign by up to three players on national signing day. Starting next season, programs will be able to give out scholarships to more players than were lost the year before.

But the NCAA will still enforce its 85-scholarship rule. If teams oversign and do not suffer any attrition, the NCAA will penalize the school two scholarships in the next recruiting class for every scholarship player a team has over its limit.

That is an unlikely problem for Big Ten schools in the near future, though, according to Northwestern coach Randy Walker.

“I don’t think anybody in the Big Ten has been at 85 for a long time,” Walker said. “Really, nobody is. And some are a lot lower than others.”

Walker said the Wildcats have about 15 unused scholarships this season. Like every school, NU has its share of turnover — take, for example, sophomore defensive tackle Thomas Derricks, who left the team for personal reasons during the summer.

But according to Walker, the Cats face a unique challenge in keeping players on the field for four years because they frequently complete degrees before their eligibility is up.

“The vast majority of our guys graduate in four years,” Walker said. “It’s unprecedented, really, in Division I football. But it happens here.”

And when players have a degree, many of them choose to forego a final football season to put it to use right away.

“We will have guys who have a fifth year of eligibility, and every year there’s several of those guys who choose not to take it,” Walker said. “And I understand. At some point the game is over, and there’s a greater future somewhere else.”

Walker knows that isn’t the case at most schools.

“The vast majority of institutions don’t graduate their kids in four years,” Walker said. “So many kids need fifth years to finish their degrees. And let’s be honest, a lot of kids go to school just wanting to play pro football. I think most of our kids come here for an education.”

Many Big Ten schools will start oversigning with hopes of making use of all their scholarships next season, and NU hopes to be among them. But Walker knows that there are no sure bets when it comes to maxing out a roster.

“You don’t hold onto 85 guys forever,” Walker said.