Risk factor

In epidemiology, a risk factor is a variable associated with an increased risk of disease or infection. Sometimes, determinant is also used, being a variable associated with either increased or decreased risk.

Correlation vs causation
Risk factors or determinants are correlational and not necessarily causal, because correlation does not imply causation. For example, being young cannot be said to cause measles, but young people are more at risk as they are less likely to have developed immunity during a previous epidemic. Statistical methods would be used in a less clear cut case to decide what level of risk the risk factor would have to present to be able to say the risk factor is linked to the disease (for example in a study of the link between smoking and lung cancer). Even then, no amount of statistical analysis could prove that the risk factor causes the disease; this could only be proven using direct methods such as a medical explanation of the disease's roots.

Terms of description
Mainly taken from risk factors for breast cancer, risk factors can be described in terms of, for example:
 * Relative risk, such as "A woman is more than 100 times more likely to develop breast cancer in her 60s than in her 20s. "
 * Fraction of incidences occurring in the group having the property of or being exposed to the risk factor, such as "99% of breast cancer cases are diagnosed in women "
 * Increase in incidence in the exposed group, such as "each daily alcoholic beverage increases the incidence of breast cancer by 11 cases per 1000 women "
 * Hazard ratio, such as "an increase in both total and invasive breast cancers in women randomized to receive estrogen and progestin for an average of 5 years, with a hazard ratio of 1.24 compared to controls"

Example
The following example of a risk factor is described in terms of the relative risk it confers, which is evaluated by comparing the risk of those exposed to the potential risk factor to those not exposed. Let's say that at a wedding, 74 people ate the chicken and 22 of them were ill, while of the 35 people who had the fish or vegetarian meal only 2 were ill. Did the chicken make the people ill?



Risk = \frac {\mbox{number of persons experiencing event (food poisoning)}} {\mbox{number of persons exposed to risk factor (food)}} $$

So the chicken eaters' risk = 22/74 = 0.297 And non-chicken eaters' risk = 2/35 = 0.057.

Those who ate the chicken had a risk over five times as high as those who did not, that is, a relative risk of more than five. This suggests that eating chicken was the cause of the illness, but this is not proof.

General determinants
The risk of an outcome usually depends on an interplay between multiple determinants. When performing epidemiological studies to evaluate one or more determinants for a specific outcome, the other determinants may act as confounding factors, and need to be controlled for, e.g. by stratification. The potentially confounding determinants varies with what outcome is studied, but the following general determinants are common to most epidemiological associations, and are the determinants most commonly controlled for in epidemiological studies:
 * Age
 * Sex or gender
 * Geographic location
 * Ethnicity
 * Genetic predisposition
 * Occupation
 * Social status
 * Level of chronic stress
 * Diet
 * Level of physical exercise
 * Alcohol consumption and tobacco smoking
 * Other diseases and conditions, commonly diabetes mellitus
 * Other social determinants of health

Risk marker
A risk marker is a variable that is quantitatively associated with a disease or other outcome, but direct alteration of the risk marker does not necessarily alter the risk of the outcome.

History
The term "risk factor" was first coined by former Framingham Heart Study Director Dr. William B. Kannel a 1961 article in Annals of Internal Medicine.