Anders Huitfeldt , postdoctoral scholar Meta-Research Innovation Center at Stanford, Stanford University School of Medicine, Palo Alto, CA, USA Correspondence to: A Huitfeldt ahuitfel{at}stanford.edu

Anders Huitfeldt argues that the answer depends on your definition of “risk factor” and calls for greater clarity in research

The risk factor approach to epidemiology was introduced by the Framingham Heart Study investigators,1 2 who first alluded to the idea in 1951.3 The first use of the term “factor of risk” appeared in 1961,4 but it was not precisely defined. The resulting semantic confusion has hindered precise communication about study design and data analysis. To illustrate the problem, let us suppose that you want to study the causes and distribution of personal wealth. You have a secretive friend, and, among other questions, you are interested in knowing whether he is a millionaire. You are aware that there are some attributes, or risk factors, that are thought to be linked to being a millionaire. You decide to investigate.