Risk factor

In epidemiology, a risk factor is a variable associated with an increased risk of disease or infection[1]:38. When evidence is found the term determinant is used as 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 prove causation. For example, being young cannot be said to cause measles, but young people have a higher rate of measles because they are less likely to have developed immunity during a previous epidemic. Statistical methods are frequently used to assess the strength of an association and to provide causal evidence (for example in the study of the link between smoking and lung cancer). Statistical analysis along with the biological sciences can establish that risk factors are causal. Some prefer the term risk factor to mean causal determinants of increased rates of disease, and for unproven links to be called possible risks, associations, etc.

When done thoughtfully and based on research, identification of risk factors can be a strategy for medical screening.[2]

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.[3]"
  • 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[4]"
  • 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[5]"
  • 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"[6]


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?

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 probability of an outcome usually depends on an interplay between multiple associated variables. 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 confounders are common to most epidemiological associations, and are the determinants most commonly controlled for in epidemiological studies:

  • Age (0 to 1.5 years for infants, 1.5 to 6 years for young children, etc.)
  • Sex or gender (Male or female)[7]:20
  • Ethnicity (Based on race)[7]:21

Other less commonly adjusted for possible confounders include:

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. For example, driving-while-intoxicated (DWI) history is a risk marker for pilots as epidemiologic studies indicate that pilots with a DWI history are significantly more likely than their counterparts without a DWI history to be involved in aviation crashes.[8]


The term "risk factor" was first coined by former Framingham Heart Study Director, Dr. William B. Kannel in a 1961 article in Annals of Internal Medicine.[9]

See also


  1. ^ a b Parritz, Robin Hornik,. Disorders of childhood : development and psychopathology. Troy, Michael F. (Michael Francis), (Third edition ed.). Boston, MA. ISBN 9781337098113. OCLC 960031712.CS1 maint: Multiple names: authors list (link) CS1 maint: Extra text (link)
  2. ^ Wald, N J; Hackshaw, A K; Frost, C D (1999). "When can a risk factor be used as a worthwhile screening test?". BMJ. 319 (7224): 1562–1565. doi:10.1136/bmj.319.7224.1562. ISSN 0959-8138. PMC 1117271.
  3. ^ Margolese, Richard G, Bernard Fisher, Gabriel N Hortobagyi, and William D Bloomer (2000). "118". In Bast RC, Kufe DW, Pollock RE, et al. (eds.). Cancer Medicine (e.5 ed.). Hamilton, Ontario: B.C. Decker. ISBN 1-55009-113-1. Retrieved 27 January 2011.CS1 maint: Multiple names: authors list (link)
  4. ^ Giordano SH, Cohen DS, Buzdar AU, Perkins G, Hortobagyi GN (July 2004). "Breast carcinoma in men: a population-based study". Cancer. 101 (1): 51–7. doi:10.1002/cncr.20312. PMID 15221988.
  5. ^ Allen NE, Beral V, Casabonne D, et al. (March 2009). "Moderate alcohol intake and cancer incidence in women". Journal of the National Cancer Institute. 101 (5): 296–305. doi:10.1093/jnci/djn514. PMID 19244173.
  6. ^ Heiss, G.; Wallace, R.; Anderson, G. L.; Aragaki, A.; Beresford, S. A. A.; Brzyski, R.; Chlebowski, R. T.; Gass, M.; Lacroix, A. (2008). "Health Risks and Benefits 3 Years After Stopping Randomized Treatment with Estrogen and Progestin". JAMA: The Journal of the American Medical Association. 299 (9): 1036–45. doi:10.1001/jama.299.9.1036. PMID 18319414.
  7. ^ a b Mash, Eric J., (2019). Abnormal child psychology. Wolfe, David A. (David Allen), 1951- (Seventh edition ed.). Boston, MA. ISBN 9781337624268. OCLC 1022139949.CS1 maint: Multiple names: authors list (link) CS1 maint: Extra text (link)
  8. ^ Li G, Baker SP, Qiang Y, Grabowski JG, McCarthy ML.Driving-while-intoxicated history as a risk marker for general aviation pilots. Accid Anal Prev. 2005;37(1):179-84./McFadden KL. Driving while intoxicated (DWI) convictions and job-related flying performance – a study of commercial air safety. J Oper Res Soc. 1998;49:28–32
  9. ^ Husten, Larry (23 August 2011). "William Kannel, Former Director of the Framingham Heart Study, Dead at 87". Forbes.

Further reading


Arteriosclerosis is the thickening, hardening, and loss of elasticity of the walls of arteries. This process gradually restricts the blood flow to one's organs and tissues and can lead to severe health risks brought on by atherosclerosis, which is a specific form of arteriosclerosis caused by the buildup of fatty plaques, cholesterol, and some other substances in and on the artery walls. It can be brought on by smoking, a bad diet, or many genetic factors. Atherosclerosis is the primary cause of coronary artery disease (CAD) and stroke, with multiple genetic and environmental contributions. Genetic-epidemiologic studies have identified a surprisingly long list of genetic and non-genetic risk factors for CAD. However, such studies indicate that family history is the most significant independent risk factor.

Atrial tachycardia

Atrial tachycardia is a type of heart rhythm problem in which the heart's electrical impulse comes from an ectopic pacemaker (that is, an abnormally located cardiac pacemaker) in the upper chambers (atria) of the heart, rather than from the sinoatrial node, the normal origin of the heart's electrical activity. Atrial tachycardias can exhibit very regular (consistent) heart rates ranging typically from 140 to 220 beats per minute.

As with any other form of tachycardia (rapid heart beat), the underlying mechanism can be either the rapid discharge of an abnormal focus, the presence of a ring of cardiac tissue that gives rise to a circle movement (reentry), or a triggered rapid rhythm due to other pathological circumstances (as would be the case with some drug toxicities, such as digoxin toxicity). Atrial tachycardia is a risk factor for atrial fibrillation, as the rapid rhythm can trigger or degrade into the lack of a rhythm. All atrial tachycardias are by definition supraventricular tachycardias.

Forms of atrial tachycardia (ATach) include multifocal atrial tachycardia (MAT), ectopic atrial tachycardia (EAT), unifocal atrial tachycardia (UAT), and paroxysmal atrial tachycardia (PAT). The taxonomy varies somewhat between users (regarding names that mean the same versus those that label subsets). The codification of the terms "first detected", "paroxysmal", "persistent", and "permanent" in the classification of atrial fibrillation should be compared for reference.

Attributable fraction among the exposed

In epidemiology, attributable fraction among the exposed (AFe) is the proportion of incidents in the exposed group that are attributable to the risk factor. Term attributable risk percent among exposed is used if the fraction is expressed as a percentage. It is calculated as , where is the incidence in the exposed group, is the incidence in the unexposed group, and is the relative risk.

It is used when an exposure increases the risk, as opposed to reducing it, in which case its symmetrical notion is preventable fraction among the unexposed.

Attributable fraction for the population

In epidemiology, attributable fraction for the population (AFp) is the proportion of incidents in the population that are attributable to the risk factor. Term attributable risk percent for the population is used if the fraction is expressed as a percentage. It is calculated as , where is the incidence in the population, and is the incidence in the unexposed group.

Equivalently it can be calculated as , where is the exposed proportion of the population and is the relative risk not adjusted for confounders.

It is used when an exposure increases the risk, as opposed to reducing it, in which case its symmetrical notion is preventable fraction for the population.

Behavioral Risk Factor Surveillance System

The Behavioral Risk Factor Surveillance System (BRFSS) is a United States health survey that looks at behavioral risk factors. It is run by Centers for Disease Control and Prevention and conducted by the individual state health departments. The survey is administered by telephone and is the world's largest such survey. In 2009, the BRFSS began conducting surveys by cellular phone in addition to traditional “landline” telephones.


Clonorchiasis is an infectious disease caused by the Chinese liver fluke, Clonorchis sinensis, and two related species.

Clonorchiasis is a known risk factor for the development of cholangiocarcinoma, a neoplasm of the biliary system.

Symptoms of opisthorchiasis caused by Opisthorchis viverrini and by Opisthorchis felineus are indistinguishable from clonorchiasis caused by Clonorchis sinensis, so the disease by these three parasites should be referred as clonorchiasis.

Dependent and independent variables

In mathematical modeling, statistical modeling and experimental sciences, the values of dependent variables depend on the values of independent variables. The dependent variables represent the output or outcome whose variation is being studied. The independent variables, also known in a statistical context as regressors, represent inputs or causes, that is, potential reasons for variation. In an experiment, any variable that the experimenter manipulates can be called an independent variable. Models and experiments test the effects that the independent variables have on the dependent variables. Sometimes, even if their influence is not of direct interest, independent variables may be included for other reasons, such as to account for their potential confounding effect.


Diverticulitis, specifically colonic diverticulitis, is a gastrointestinal disease characterized by inflammation of abnormal pouches—diverticula—which can develop in the wall of the large intestine. Symptoms typically include lower abdominal pain of sudden onset, but onset may also occur over a few days. In North America and Europe the abdominal pain is usually on the left lower side (sigmoid colon), while in Asia it is usually on the right (ascending colon). There may also be nausea; and diarrhea or constipation. Fever or blood in the stool suggests a complication. Repeated attacks may occur.The causes of diverticulitis are uncertain. Risk factors may include obesity, lack of exercise, smoking, a family history of the disease, and use of nonsteroidal anti-inflammatory drugs (NSAIDs). The role of a low fiber diet as a risk factor is unclear. Having pouches in the large intestine that are not inflamed is known as diverticulosis. Inflammation occurs in between 10% and 25% at some point in time, and is due to a bacterial infection. Diagnosis is typically by CT scan, though blood tests, colonoscopy, or a lower gastrointestinal series may also be supportive. The differential diagnosis includes irritable bowel syndrome.Preventive measures include altering risk factors such as obesity, inactivity, and smoking. Mesalazine and rifaximin appear useful for preventing attacks in those with diverticulosis. Avoiding nuts and seeds as a preventive measure is no longer recommended since there is no evidence these play a role in initiating inflammation in diverticula. For mild diverticulitis, antibiotics by mouth and a liquid diet are recommended. For severe cases, intravenous antibiotics, hospital admission, and complete bowel rest may be recommended. Probiotics are of unclear use. Complications such as abscess formation, fistula formation, and perforation of the colon may require surgery.The disease is common in the Western world and uncommon in Africa and Asia. In the Western world about 35% of people have diverticulosis while it affects less than 1% of those in rural Africa, and 4 to 15% of those may go on to develop diverticulitis. The disease becomes more frequent with age, being particularly common in those over the age of 50. It has also become more common in all parts of the world. In 2003 in Europe, it resulted in approximately 13,000 deaths. It is the most frequent anatomic disease of the colon. Costs associated with diverticular disease were around US$2.4 billion a year in the United States in 2013.

Endometrial hyperplasia

Endometrial hyperplasia is a condition of excessive proliferation of the cells of the endometrium, or inner lining of the uterus.

Most cases of endometrial hyperplasia result from high levels of estrogens, combined with insufficient levels of the progesterone-like hormones which ordinarily counteract estrogen's proliferative effects on this tissue. This may occur in a number of settings, including obesity, polycystic ovary syndrome, estrogen producing tumours (e.g. granulosa cell tumour) and certain formulations of estrogen replacement therapy. Endometrial hyperplasia is a significant risk factor for the development or even co-existence of endometrial cancer, so careful monitoring and treatment of women with this disorder is essential.


Fenpropathrin (brand names Danitol, Meothrin), or fenopropathrin, is a widely used pyrethroid insecticide in agriculture and household.A person developed Parkinson's disease after six months of daily exposure to fenpropathrin, and animal tests subsequently revealed that the compound is a dopaminergic neurotoxin. It has thus been implicated as an environmental risk factor for Parkinson's disease.


Hidradenitis is any disease in which the histologic abnormality is primarily an inflammatory infiltrate around the eccrine glands. This group includes neutrophilic eccrine hidradenitis and recurrent palmoplantar hidradenitis.It can also be defined more generally as an inflammation of sweat glands.Hidradenitis suppurativa is a chronic cutaneous condition originally thought to be primarily characterized by suppurative inflammation of the apocrine sweat glands. However, recent evidence supports that the primary event is follicular hyperkeratosis and obstruction.


Homocysteine is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (-CH2-). It is biosynthesized from methionine by the removal of its terminal Cε methyl group. Homocysteine can be recycled into methionine or converted into cysteine with the aid of certain B-vitamins.

A high level of homocysteine in the blood (hyperhomocysteinemia) makes a person more prone to endothelial cell injury, which leads to inflammation in the blood vessels, which in turn may lead to atherogenesis, which can result in ischemic injury. Hyperhomocysteinemia is therefore a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood.

Hyperhomocysteinemia has been correlated with the occurrence of blood clots, heart attacks and strokes, though it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has also been associated with early pregnancy loss and with neural tube defects.

Number needed to harm

The number needed to harm (NNH) is an epidemiological measure that indicates how many persons on average need to be exposed to a risk factor over a specific period to cause harm in an average of one person who would not otherwise have been harmed. It is defined as the inverse of the absolute risk increase, and computed as , where is the incidence in the treated (exposed) group, and is the incidence in the control (unexposed) group. Intuitively, the lower the number needed to harm, the worse the risk factor, with 1 meaning that every exposed person is harmed.

NNH is similar to number needed to treat (NNT), where NNT usually refers to a therapeutic intervention and NNH to a detrimental effect or a risk factor.

Ocular hypertension

Ocular hypertension is the presence of elevated fluid pressure inside the eye (intraocular pressure), usually with no optic nerve damage or visual field loss.For most individuals, the normal range of introcular pressure is between 10 mmHg and 21 mmHg. Elevated intraocular pressure is an important risk factor for glaucoma. The Ocular Hypertension Treatment Study, a large, multicentered, randomized clinical trial, determined that topical ocular hypotensive medication delays or prevents the onset of Primary Open-Angle Glaucoma. Accordingly, most individuals with consistently elevated intraocular pressures of greater than 21mmHg, particularly if they have other risk factors, are treated in an effort to prevent vision loss from glaucoma.

Population Impact Measures

Population Impact Measures (PIMs) are biostatistical measures of risk and benefit used in epidemiological and public health research. They are used to describe the impact of health risks and benefits in a population, to inform health policy.Frequently used measures of risk and benefit identified by Jerkel, Katz and Elmore, describe measures of risk difference (attributable risk), rate difference (often expressed as the odds ratio or relative risk), Population Attributable Risk (PAR), and the relative risk reduction, which can be recalculated into a measure of absolute benefit, called the Number needed to treat. Population Impact Measures are an extension of these statistics, as they are measures of absolute risk at the population level, which are calculations of number of people in the population who are at risk to be harmed, or who will benefit from Public Health interventions.

They are measures of absolute risk and benefit, producing numbers of people who will benefit from an intervention or be at risk from a risk factor within a particular local or national population. They provide local context to previous measures, allowing policy-makers to identify and prioritise the potential benefits of interventions on their own population. They are simple to compute, and contain the elements to which policy-makers would have to pay attention in the commissioning or improvement of services. They may have special relevance for local policy-making. They depend on the ability to obtain and use local data, and by being explicit about the data required may have the added benefit of encouraging the collection of such data.

Risk dominance

Risk dominance and payoff dominance are two related refinements of the Nash equilibrium (NE) solution concept in game theory, defined by John Harsanyi and Reinhard Selten. A Nash equilibrium is considered payoff dominant if it is Pareto superior to all other Nash equilibria in the game.1 When faced with a choice among equilibria, all players would agree on the payoff dominant equilibrium since it offers to each player at least as much payoff as the other Nash equilibria. Conversely, a Nash equilibrium is considered risk dominant if it has the largest basin of attraction (i.e. is less risky). This implies that the more uncertainty players have about the actions of the other player(s), the more likely they will choose the strategy corresponding to it.

The payoff matrix in Figure 1 provides a simple two-player, two-strategy example of a game with two pure Nash equilibria. The strategy pair (Hunt, Hunt) is payoff dominant since payoffs are higher for both players compared to the other pure NE, (Gather, Gather). On the other hand, (Gather, Gather) risk dominates (Hunt, Hunt) since if uncertainty exists about the other player's action, gathering will provide a higher expected payoff. The game in Figure 1 is a well-known game-theoretic dilemma called stag hunt. The rationale behind it is that communal action (hunting) yields a higher return if all players combine their skills, but if it is unknown whether the other player helps in hunting, gathering might turn out to be the better individual strategy for food provision, since it does not depend on coordinating with the other player. In addition, gathering alone is preferred to gathering in competition with others. Like the Prisoner's dilemma, it provides a reason why collective action might fail in the absence of credible commitments.

Risk factors for breast cancer

Risk factors for breast cancer may be divided into preventable and non-preventable. Their study belongs in the field of epidemiology. Breast cancer, like other forms of cancer, can result from multiple environmental and hereditary risk factors. The term "environmental", as used by cancer researchers, means any risk factor that is not genetically inherited.

For breast cancer, the list of environmental risk factors includes the individual person's development, exposure to microbes, "medical interventions, dietary exposures to nutrients, energy and toxicants, ionizing radiation, and chemicals from industrial and agricultural processes and from consumer products...reproductive choices, energy balance, adult weight gain, body fatness, voluntary and involuntary physical activity, medical care, exposure to tobacco smoke and alcohol, and occupational exposures, including shift work" as well as "metabolic and physiologic processes that modify the body's internal environment." Some of these environmental factors are part of the physical environment, while others (such as diet and number of pregnancies) are primarily part of the social, cultural, or economic environment.Although many epidemiological risk factors have been identified, the cause of any individual breast cancer is most often unknowable. Epidemiological research informs the patterns of breast cancer incidence across certain populations, but not in a given individual. Approximately 5% of new breast cancers are attributable to hereditary syndromes, and well-established risk factors accounts for approximately 30% of cases.

Run (cricket)

In cricket, a run is the unit of scoring. The team with the most runs wins in many versions of the game, and always draws at worst (see result), except for some results decided by the Duckworth–Lewis method. A single run (known as a "single") is scored when a batsman (known as the "striker") has hit the ball with the bat and directed it away from the fielders so that both the striker and non-striker partner are able to run the length (22 yards) of the pitch, crossing each other and arriving safely at the other end of the pitch, before the fielders can retrieve the ball.

Depending on how long it takes the fielding team to recover the ball, the batsmen may run more than once. Each completed run increments the scores of both the team and the striker. A batsman may also score 4 or 6 runs by striking the ball to the boundary. The team's total score in the innings is the aggregate of all its batsmen's individual scores plus any extras and penalties. To complete a run, both batsmen must make their ground, with some part of their person or bat behind the popping crease at the other end of the pitch. Attempting a run carries a risk factor because either batsman can be run out, (one method of dismissal), if the fielding side can break the wicket with the ball before the batsman has completed the run.

Volatility risk

Volatility risk is the risk of a change of price of a portfolio as a result of changes in the volatility of a risk factor. It usually applies to portfolios of derivatives instruments, where the volatility of its under lyings is a major influencer of prices.

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