Causation vs. Association – the Basics
To those of us who are not scientists or epidemiologists two of the most confusing concepts in the universe are association and causation. Many of us are helping parents age as gracefully as possible in the midst of devastating diseases and are deeply frustrated that we cannot sort out the factors associated with an illness from the factors that cause it. For example, age is associated with many of the conditions that our parents face, dementia, for instance, but it doesn’t cause these diseases outright.
When we experience disease in our families — in mine dementia, congestive heart failure, and bi-polar disorder — it is natural after nursing and maybe losing loved ones who suffered severe illnesses — to be passionate about cures and prevention. I’ve observed personally how earnestly we want to prevent a recurrence of an illness. Moreover, after a family member has died, it feels good, almost as a memorial, to support research that is seeking a cure. That’s probably why so many of us in the non-scientific community spend weekends running, walking, cycling, and participating in countless other fund-raising activities, helping to support medical research.
Which brings me to association, causation, dementia, and Alzheimer’s. Early this week a National Institutes of Health (NIH) Alzheimer’s disease conference, sponsored by the NIH Office of Medical Applications of Research and other related agencies, set out to reach a consensus on the state of Alzheimer’s research. A panel of experts and respected scientists, though not those with a vested interest in dementia research, was convened to examine what is known and not known about the causes of Alzheimer’s. Furthermore the group examined evidence to determine whether factors such as exercise, special diets, and cognitive activities have any effect on prevention.
After meeting for three days, the panel members concluded that the many preventive measures those of us in the general population hear about and embrace — the exercises, crossword puzzles, and healthy diets — have not been adequately documented as factors that prevent Alzheimer’s. While many are promoted and sold in the media as prevention activities, there is no solid scientific evidence that they help stop the development of the disease. The panel found that research so far has loosely associated these and other behaviors with a lower incidence of dementia — call it association lite — but nothing is more definitive. According to an NIH press release, “The panel determined that there is currently no evidence of even moderate scientific quality supporting the association of any modifiable factor with reduced risk of Alzheimer’s disease and cognitive decline.”
Will not engaging in these activities cause the disease? No one can say. However, the panel pointed out that the healthy behaviors that continue to be weakly associated with the prevention of Alzheimer’s are indeed positive, and there is scientific evidence that they contribute to overall health and help people stay healthy and mentally alert. Causation, that is identifying the specific factors that cause and contribute to Alzheimer’s disease, remains elusive.
So for the present scientists will continue looking for the causes of Alzheimer’s and other dementas. For now, studies that monitor large numbers of people for a long period of time — we adult children will probably make great contributions to this endeavor — may yield the most insights.
- Independent Panel Finds Insufficient Evidence to Support Preventive Measures for Alzheimer’s Disease conference press release
- Conference Consensus Statement (long)
- Understanding Medical Research at Medline
- Frequently Asked Questions About Clinical Research at the National Human Genome Research Institute
- Alzheimer’s Disease Education and Referral (ADEAR) Center National Institute on Aging
- The Environment and Disease: Association or Causation? The 1965 paper by Sir Austin Bradford Hill that started the discussion about association and causation. Challenging reading but worth it if you are interested in this topic.