Intended for healthcare professionals

Feature Christmas 2016: Food for Thought

Is caviar a risk factor for being a millionaire?

BMJ 2016; 355 doi: (Published 09 December 2016) Cite this as: BMJ 2016;355:i6536
  1. Anders Huitfeldt, postdoctoral scholar
  1. Meta-Research Innovation Center at Stanford, Stanford University School of Medicine, Palo Alto, CA, USA
  1. Correspondence to: A Huitfeldt ahuitfel{at}

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.

What is a risk factor?

The first step is to choose your definition of risk factor. Clinical research can generally be divided into four broad objectives based on the intended use of the information obtained by the study: diagnosis, prognosis, treatment effects, and aetiology. Each of these research objectives is associated with a different definition. Table 1 gives examples of how these four definitions of risk factor are used in the scientific literature and shows how each definition describes a different relation between the dependent variable and the independent variable.

View this table:
Table 1

Objectives of clinical research and associated definitions of risk factor

A variable may qualify as a risk factor under more than one definition of the term. For example, cholesterol is believed to be a risk factor for heart disease under each of the four definitions. However, it is generally not plausible to assume that a variable that is a risk factor according to one definition will always be a risk factor under the …

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