This study at best is biased at worst is agenda driven. It's like doing a study on 300 lb. plus people that died of heart disease at an early age and ate a lot of fast food. Then saying we need to get rid of all fast food restaurants.
My problem with a lot of the CTE studies is they have a laser focus on football because it gives them the most bang for their buck. If you say football causes CTE it's always bound to get a headline. If you say soccer not so much.
This feeds into the sky is falling types fears. Yes CTE is real but studies like this do more harm then good. Unless your goal is to grab a headline.
This study is not "biased" it is an observational study looking at a sample of "convenience". They report exactly how they came to look at these brains and what they found. Indeed there are no controls as this is a retrospective study looking at the clinical material that was donated. This is how science works - you first examine and report on available data. If the results are suggestive of an association (in this case between participation in football and CTE) you take these results and apply for funding to do the large and expensive research project that will be necessary to suggest a causal relationship. The gold standard of medical research, randomized controlled trials (RCTs), cannot be done for obvious ethical and logistical reasons - one cannot randomly select teenagers and randomize them to high school, college and professional football vs no football (the control arm). Perhaps the next step will be to do "case control studies" where prospective teenagers who play football are "paired" with an individual of similar characteristics (height, weight, gender, etc.) who doesn't play football. They would need to be followed over time and agree to donate their brains for examination after their deaths. This could provide much stronger information about causality but not proof. Proof requires RCTs or a large body of research, collected over time supporting the hypothesis (that the repetitive head trauma of football increases ones risk of developing CTE).
To determine the true incidence of CTE in the NFL
all present players would need to donate their brains for examination. Strong inferences could be made if a randomly selected sample of appropriate size would donate. That data would be free of the sampling bias the authors disclosed. If you are not alarmed by this report you might want to read it again. Even for a self reported sample these are crazy high positive numbers. This says nothing about the incidence of CTE in other sports - it's possible it is even higher in rugby or soccer, this only looks at football players.
Please read the last paragraph in the author's discussion and their conclusions:
This study had several limitations. First, a major limitation is ascertainment bias associated with participation in this brain donation program. Although the criteria for participation were based on exposure to repetitive head trauma rather than on clinical signs of brain trauma, public awareness of a possible link between repetitive head trauma and CTE may have motivated players and their families with symptoms and signs of brain injury to participate in this research. Therefore, caution must be used in interpreting the high frequency of CTE in this sample, and estimates of prevalence cannot be concluded or implied from this sample. Second, the VA-BU-CLF brain bank is not representative of the overall population of former players of American football; most players of American football have played only on youth or high school teams, but the majority of the brain bank donors in this study played at the college or professional level. Additionally, selection into brain banks is associated with dementia status, depression status, marital status, age, sex, race, and education.
36Third, this study lacked a comparison group that is representative of all individuals exposed to American football at the college or professional level, precluding estimation of the risk of participation in football and neuropathological outcomes.
Conclusions
In a convenience sample of deceased football players who donated their brains for research, a high proportion had neuropathological evidence of CTE, suggesting that CTE may be related to prior participation in football.