Signs of a degenerative brain disease were widespread among a sample of donated brains of former football players, researchers reported Tuesday in the Journal of the American Medical Association.

The finding bolsters the connection between playing American football and developing Chronic traumatic encephalopathy (CTE), which is linked to repeated blows to the head and was first described in boxers. However, the large study provides little new information about the disease, its progression, or prevalence.

The bank of 202 former football players’ brains is a “convenience sample,” meaning it’s a biased sampling not representative of football players overall. Instead, players and their families donated the brains after players experienced symptoms connected with CTE during life or the players were suspected or considered at risk of developing CTE. The athletes represented in the sample reported much higher rates of CTE symptoms in life than those found in surveys of living, retired National Football League (NFL) players. Also, the study only had pathology data from one time point—after death—so progression of the disease couldn’t be examined. And, last, the study did not include a sampling of brains from people who were not exposed to football—a control group.