Risk factor is old concept with new level of acceptance

January 15, 2008
Nancy Groves

The medical profession was slow to adopt the concept of the risk factor, which was born from the marriage of statistics and probability theory. Gradually, however, it has become central to medicine and science and will assume even more importance in coming years.

Key Points

In that lecture, Dr. Wilson, chancellor, University of Colorado Denver (UCD), and professor of ophthalmology, UCD School of Medicine, explored the birth, evolution, and practice of the risk factor, calling it a concept that will continue to be fundamentally important in the understanding and management of glaucoma and other chronic diseases.

He defined a risk factor as a pattern of behavior or physical characteristic of a group of individuals that increases the probability of the future occurrence of one or more diseases in that group relative to comparable groups without the behavior or characteristic. Probability is a key word in that definition, and probability theory was one of the two disciplines that gave rise to the concept of the risk factor, Dr. Wilson said.


Probability often was applied in games of chance before its much later adoption by the medical and scientific fields, Dr. Wilson said. Statistics, the other cornerstone of probability, emerged in the mid 1700s primarily as a means to quantify aspects of social life, such as population estimates or the number of deaths in a given period, he added, and it later was used in the development of life tables for annuities.

"This marriage of probability theory and statistics was absolutely critical in furthering this concept of risk but was not accepted in medical science until the 20th century, primarily because it rejected determinism, which was the prevailing concept at the time.

"The basic tenet of probability is that no matter what the process, you can never expect to get the same effect every time without fail. On the other hand, the physical scientists of this era, led by people like Sir Isaac Newton, felt that you could predict a specific outcome with absolute certainty," he added. Inability to do so simply reflected a limitation in current knowledge and the need for more research, Dr Wilson said.

The medical scientists of that era wanted to follow the same rigorous process as the physical scientists, so they too adhered to determinism over probability. It was the insurance industry, followed by the social sciences, that propelled probability theory into the mainstream.

The birth of the concept of risk factors was facilitated by several developments, such as the development and adoption of probability and statistics as methods of quantifying risk of death and disease, acceptance of multifactorial models of disease etiology, and the shift from infectious diseases to chronic diseases as the major cause of death, Dr. Wilson said.

One of the earliest and best-known uses of statistics in medicine occurred during the London cholera epidemic of 1854, when John Snow, MD, used statistics to prove his theory that cholera reproduced in the body and was spread by contaminated water. The commonly accepted idea at the time was that cholera was transmitted through inhalation of contaminated vapors.

While investigating the epidemic, Dr. Snow plotted cases of cholera and discovered that the disease was more likely to occur in houses with water supplied by a company that drew its water from the Thames River downstream from the city, where it was contaminated by sewage, than in houses where the water was supplied by a company that drew its water upstream. Dr. Snow also pinpointed one area where 500 deaths from cholera occurred within 10 days and advised local officials to remove the handle from the neighborhood's water pump. After that task was done, the epidemic was contained.

It was the life insurance industry that kept statistics alive in the late 19th century, however, using them to increase accuracy in predicting deaths. They began to identify medical and non-medical factors associated with death and quantifying risk based on these factors. This practice gave rise to health concepts now taken for granted, such as that of normal and abnormal blood pressure levels, Dr. Wilson said.