For the last 13 years, the N.F.L. has stood by the research, which, the papers stated, was based on a full accounting of all concussions diagnosed by team physicians from 1996 through 2001. But confidential data obtained by The Times shows that more than 100 diagnosed concussions were omitted from the studies — including some severe injuries to stars like quarterbacks Steve Young and Troy Aikman. The committee then calculated the rates of concussions using the incomplete data, making them appear less frequent than they actually were.

After The Times asked the league about the missing diagnosed cases — more than 10 percent of the total — officials acknowledged that “the clubs were not required to submit their data and not every club did.” That should have been made clearer, the league said in a statement, adding that the missing cases were not part of an attempt “to alter or suppress the rate of concussions.”

One member of the concussion committee, Dr. Joseph Waeckerle, said he was unaware of the omissions. But he added: “If somebody made a human error or somebody assumed the data was absolutely correct and didn’t question it, well, we screwed up. If we found it wasn’t accurate and still used it, that’s not a screw-up; that’s a lie.”

These discoveries raise new questions about the validity of the committee’s findings, published in 13 peer-reviewed articles and held up by the league as scientific evidence that brain injuries did not cause long-term harm to its players.