Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine. Health is a state of complete physical mental and social well-being and not merely the absence of disease or infirmity Illness (sometimes referred to as ill-health or ail) can be defined as a state of poor Health. Logic is the study of the principles of valid demonstration and Inference. Public health is "the science and art of preventing disease prolonging life and promoting health through the organised efforts and informed choices of society organisations Generally speaking preventive medicine is the part of Medicine engaged with preventing Disease rather than curing it It is considered a cornerstone methodology of public health research, and is highly regarded in evidence-based medicine for identifying risk factors for disease and determining optimal treatment approaches to clinical practice. Evidence-based medicine (EBM aims to apply Evidence gained from the Scientific method to certain parts of medical practice A disease is an abnormal condition of an organism that impairs bodily functions and can be deadly

The work of communicable and non-communicable disease epidemiologists ranges from outbreak investigation to study design, data collection and analysis including the development of statistical models to test hypotheses and the documentation of results for submission to peer-reviewed journals. Outbreak is a classification used in Epidemiology to describe a small localized group of people or organisms infected with a disease Epidemiologists may draw on a number of other scientific disciplines such as biology in understanding disease processes and social science disciplines including sociology and philosophy in order to better understand proximate and distal risk factors. Sociology (from Latin: socius "companion" and the suffix -ology "the study of" from Greek λόγος lógos "knowledge" Philosophy is the study of general problems concerning matters such as existence knowledge truth beauty justice validity mind and language

Contents 1 Etymology 2 History 3 The profession 4 The practice 5 As causal inference 5.1 Bradford-Hill criteria 5.2 Legal interpretation 6 Advocacy 7 Population-based health management 8 Types of studies 8.1 Case series 8.2 Case control studies 8.3 Cohort studies 8.4 Outbreak investigation 9 Validity: precision and bias 9.1 Random error 9.2 Systematic error 9.3 Selection bias 10 Journals 10.1 General journals 10.2 Specialty journals 11 Areas 11.1 By physiology/disease 11.2 By methodological approach 12 See also 13 Footnotes 14 Sources 15 External links

Etymology

Epidemiology, "the study of what is upon the people," is derived from the Greek terms epi = upon, among; demos = people, district; logos = study, word, discourse; suggesting that it applies only to human populations. But the term is widely used in studies of zoological populations (veterinary epidemiology), although the term 'epizoology' is available, and it has also been applied to studies of plant populations (botanical epidemiology). Epizoology is the study of Disease patterns within animal populations ^[1]

History

The Greek physician Hippocrates is sometimes said to be the father of epidemiology. Hippocrates of Cos II or Hippokrates of Kos ( ca. 460 BC – ca He is the first person known to have examined the relationships between the occurrence of disease and environmental influences. He coined the terms endemic (for diseases usually found in some places but not in others) and epidemic (for disease that are seen at some times but not others). In Epidemiology, an Infection is said to be endemic (from Greek en- in or within + demos people in a Population when In Epidemiology, an epidemic (from Greek epi- upon + demos people is a classification of a disease that appears as new cases in a ^[2]

One of the earliest theories on the origin of disease was that it was primarily the fault of human luxury. This was expressed by philosophers such as Plato^[3] and Rousseau,^[4] and social critics like Jonathan Swift. Biography Early life Birth and family Plato was born in Athens Greece ^[5]

In the medieval Islamic world, physicians discovered the contagious nature of infectious disease. An infectious disease is a clinically evident Disease resulting from the presence of Pathogenic microbial agents including Pathogenic viruses Pathogenic In particular, the Persian physician Avicenna, considered a "father of modern medicine,"^[6] in The Canon of Medicine (1020s), discovered the contagious nature of tuberculosis and sexually transmitted disease, and the distribution of disease through water and soil. layout and formatting it should ensure no clashes with the top of the infobox TemplateInfobox Muslim scholars --> ( Persian /ابو علی الحسین ابن عبدالله ابن سینا (born The Canon of Medicine ( Arabic: القانون في الطب Al-Qanun fi al-Tibb " The Law of Medicine " Persian Tuberculosis (abbreviated as TB for tubercle bacillus or T u' b' erculosis Bacillus --> is a common A sexually transmitted disease ( STD) or venereal disease ( VD) is an illness that has a significant probability of transmission between Humans A disease is an abnormal condition of an organism that impairs bodily functions and can be deadly Water is a common Chemical substance that is essential for the survival of all known forms of Life. Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel ^[7] Avicenna stated that bodily secretion is contaminated by foul foreign earthly bodies before being infected. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have ^[8] He introduced the method of quarantine as a means of limiting the spread of contagious disease. For other uses see Quarantine (disambiguation Quarantine is voluntary or compulsory isolation typically to contain the spread of something ^[9] He also used the method of risk factor analysis, and proposed the idea of a syndrome in the diagnosis of specific diseases. A risk factor is a variable associated with an increased risk of Disease or Infection. In Medicine and Psychology, the term syndrome refers to the association of several clinically recognizable features signs (observed by a physician Diagnosis is the identification by Process of elimination, of the nature of anything ^[10]

When the Black Death (bubonic plague) reached Al Andalus in the 14th century, Ibn Khatima hypothesized that infectious diseases are caused by small "minute bodies" which enter the human body and cause disease. Bubonic plague is the best-known manifestation of the bacterial disease plague, caused by the bacterium Yersinia pestis (formerly known as Another 14th century Andalusian-Arabian physician, Ibn al-Khatib (1313–1374), wrote a treatise called On the Plague, in which he stated how infectious disease can be transmitted through bodily contact and "through garments, vessels and earrings. "^[8]

In the middle of the 16th century, a famous Italian doctor from Florence named Girolamo Fracastoro was the first to propose a theory that these very small, unseeable, particles that cause disease were alive. Florence ( Italian: Firenze Florentia and Fiorenza) is the Capital City of the Italian region of Tuscany Girolamo Fracastoro ( Fracastorius) (1478‑ August 8, 1553) was an Italian Physician, Scholar (in Mathematics They were considered to be able to spread by air, multiply by themselves and to be destroyable by fire. In this way he refuted Galen's theory of miasms (poison gas in sick people). Galen ( Greek: Γαληνός Galēnos; Latin: Claudius Galenus, Aelius Galenus, Claudius Aelius Galenus, or The miasmatic theory of disease held that Diseases such as Cholera or the Black Death were caused by a miasma (Greek language "pollution" In 1543 he wrote a book De contagione et contagiosis morbis, in which he was the first to promote personal and environmental hygiene to prevent disease. Hygiene refers to practices associated with ensuring good health and cleanliness The development of a sufficiently powerful microscope by Anton van Leeuwenhoek in 1675 provided visual evidence of living particles consistent with a germ theory of disease. Antonie Philips van Leeuwenhoek (October 24 1632 &ndash August 30 1723 was a Dutch tradesman and Scientist from Delft, the Netherlands The germ theory, also called the pathogenic theory of medicine, is a Theory that proposes that Microorganisms are the cause of many Diseases.

John Graunt, a professional haberdasher and serious amateur scientist, published Natural and Political Observations . A cluster refers to a grouping of health-related events that are related temporally and in proximity John Graunt ( April 24, 1620 - April 18, 1674) was one of the first Demographers though by profession he was a Haberdasher A haberdasher is a person who sells small articles for Sewing, such as buttons ribbons and zippers . . upon the Bills of Mortality in 1662. In it, he used analysis of the mortality rolls in London before the Great Plague to present one of the first life tables and report time trends for many diseases, new and old. London ( ˈlʌndən is the capital and largest urban area in the United Kingdom. The Great Plague (1665-1666 was a massive outbreak of Disease in England that killed 75000 to 100000 people up to a fifth of London 's population In Actuarial science, a life table (also called a mortality table or actuarial table) is a table which shows for a person at each age what the probability He provided statistical evidence for many theories on disease, and also refuted many widespread ideas on them.

Dr. John Snow is famous for the suppression of an 1854 outbreak of cholera in London's Soho district. John Snow ( 15 March 1813 &ndash 16 June 1858) was a British physician and a leader in the adoption of Anaesthesia and medical Year 1854 ( MDCCCLIV) was a Common year starting on Sunday (link will display the full calendar of the Gregorian Calendar (or a Common year Cholera, sometimes known as Asiatic cholera or epidemic cholera, is an infectious Gastroenteritis caused by the Bacterium This article is about an area of Manhattan, New York City. For the area in London UK see Soho. He identified the cause of the outbreak as a public water pump on Broad Street and had the handle removed, thus ending the outbreak. Broadwick Street (formerly Broad Street is a street in Soho, City of Westminster London. (It has been questioned as to whether the epidemic was already in decline when Snow took action. ) This has been perceived as a major event in the history of public health and can be regarded as the founding event of the science of epidemiology. Public health is "the science and art of preventing disease prolonging life and promoting health through the organised efforts and informed choices of society organisations

Other pioneers include Danish physician P. A. Schleisner, who in 1849 related his work on the prevention of the epidemic of tetanus neonatorum on the Vestmanna Islands in Iceland. Year 1849 ( MDCCCXLIX) was a Common year starting on Monday (link will display the full calendar of the Gregorian calendar (or a Common Tetanus is a medical condition that is characterized by a prolonged contraction of Skeletal muscle fibres Iceland, officially the Republic of Iceland ( ( Ísland or Lýðveldið Ísland ( Another important pioneer was Hungarian physician Ignaz Semmelweis, who in 1847 brought down infant mortality at a Vienna hospital by instituting a disinfection procedure. Hungary (Magyarország 'mɔɟɔrorsaːg) officially in English the Republic of Hungary ( Magyar Köztársaság, literally Magyar (Hungarian Republic Ignaz Philipp Semmelweis ( July 1, 1818 &ndash August 13, 1865) also Ignac Semmelweis (born Semmelweis Ignác Fülöp Year 1847 ( MDCCCXLVII) was a Common year starting on Friday (link will display the full calendar of the Gregorian Calendar (or a Common His findings were published in 1850, but his work was ill received by his colleagues, who discontinued the procedure. For the game see 1850 (board game. 1850 ( MDCCCL) was a Common year starting on Tuesday (link Disinfection did not become widely practiced until British surgeon Joseph Lister 'discovered' antiseptics in 1865 in light of the work of Louis Pasteur. Joseph Lister 1st Baron Lister, OM, FRS ( 5 April 1827 &ndash 10 February 1912) was an English surgeon Year 1865 ( MDCCCLXV) was a Common year starting on Sunday (link will display the full calendar of the Gregorian calendar (or a Common year Louis Pasteur (27 December 1822 – 28 September 1895 a French Chemist and Microbiologist, is best known for remarkable breakthroughs in the causes and

In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross, Anderson Gray McKendrick and others. Sir Ronald Ross KCB, ( 13 May 1857 – 16 September 1932) was an Indian Physician. Anderson Gray McKendrick ( September 8, 1876 - May 30, 1943) was a Scottish Physician and Epidemiologist pioneered

Another breakthrough was the 1954 publication of the results of a British Doctors Study, led by Richard Doll and Austin Bradford Hill, which lent very strong statistical support to the suspicion that tobacco smoking was linked to lung cancer. Year 1954 ( MCMLIV) was a Common year starting on Friday (link will display full 1954 Gregorian calendar) The British doctors study is the generally accepted name of a Prospective Cohort study which has been running from 1951 to 2001 and in 1956 provided convincing statistical Sir William Richard Shaboe Doll CH OBE FRS ( 28 October 1912 &ndash 24 July 2005) was a British Sir Austin Bradford Hill FRS ( July 8, 1897 - April 18, 1991) English Epidemiologist and Statistician Tobacco Smoking is the inhalation of smoke from burned dried or cured leaves of the Tobacco plant most often in the form of a Cigarette. Lung cancer is a Disease of uncontrolled Cell growth in tissues of the Lung.

• History of emerging infectious diseases

The profession

To date, few universities offer epidemiology as a course of study at the undergraduate level. The discovery of Disease -causing Pathogens is an important activity in the field of medical science as many Viruses Bacteria, Protozoa, A university is an institution of Higher education and Research, which grants Academic degrees in a variety of subjects Many epidemiologists are physicians, or hold other postgraduate degrees including a Master of Public Health (MPH), Master of Science or Epidemiology (MSc. A physician, medical practitioner or medical doctor who practices Medicine, and is concerned with maintaining or restoring human Health The Master of Public Health ( MPH or MPH) is a professional Master's degree awarded for studies in areas related to Public health. A Master of Science ( Latin: Magister Scientiæ; abbreviated MSc, M ) Doctorates include the Doctor of Public Health (DrPH), Doctor of Philosophy (PhD), Doctor of Science (ScD), or for clinically trained physicians, Doctor of Medicine (MD). A doctorate is an Academic degree that indicates the highest level of academic achievement The Doctor of Public Health ( DrPH) is an advanced Professional degree for those who intend to pursue or advance a professional practice career in Public "PhD" redirects here for other uses see PhD (disambiguation. DSc ScD SD, or DrSc are common abbreviations for the Latin Scientiæ Doctor, meaning Doctor of Science. Doctor of Medicine ( MD or MD, from the Latin Medicinæ Doctor meaning "Teacher of Medicine" is a doctoral In the United Kingdom, the title of 'doctor' is a honorary one conferred to those having attained the professional degrees of Bachelor of Medicine and Surgery (MBBS or MBChB). Bachelor of Medicine and Bachelor of Surgery, or in Latin Medicinae Baccalaureus et Baccalaureus Chirurgiae (abbreviated MB BChir, BM BCh, As public health/health protection practitioners, epidemiologists work in a number of different settings. Some epidemiologists work 'in the field', i. e. , in the community, commonly in a public health/health protection service and are often at the forefront of investigating and combating disease outbreaks. Others work for non-profit organizations, universities, hospitals and larger government entities such as the Centers for Disease Control and Prevention (CDC), the Health Protection Agency, or the Public Health Agency of Canada. The Centers for Disease Control and Prevention (or CDC) is an agency of the United States Department of Health and Human Services based in unincorporated The Health Protection Agency ( HPA) originally established as a special health authority (SpHA in 2003, is an Non-Departmental Public Body charged The Public Health Agency of Canada ( French: Agence de la santé publique du Canada) is an agency of Health Canada, a department of the

The practice

Epidemiologists employ a range of study designs from the observational to experimental and are generally categorized as descriptive, analytic (aiming to further examine known associations or hypothesized relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions). Epidemiological studies are aimed, where possible, at revealing unbiased relationships between exposures such as alcohol or smoking, biological agents, stress, or chemicals to mortality or morbidity. Exposure assessment is a branch of Environmental science that focuses on the processes that take place at the interface between the environment containing the contaminant(s An infection is the detrimental Colonization of a host Organism by a foreign Species. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. Death is the termination of the biological functions that define living Organisms It refers both to a specific In Medicine, Epidemiology and Actuarial science, the term morbidity can refer to the state of poor health (from Latin Identifying causal relationships between these exposures and outcomes are important aspects of epidemiology. Modern epidemiologist use disease informatics as a tool. Disease Informatics is the application of Information Science in Defining the diseases with least error identifying most of the targets to combat a cluster of diseases

The term 'epidemiologic triad' is used to describe the intersection of Host, Agent, and Environment in analyzing an outbreak.

As causal inference

Although epidemiology is sometimes viewed as a collection of statistical tools used to elucidate the associations of exposures to health outcomes, a deeper understanding of this science is that of discovering causal relationships.

It is nearly impossible to say with perfect accuracy how even the most simple physical systems behave beyond the immediate future, much less the complex field of epidemiology, which draws on biology, sociology, mathematics, statistics, anthropology, psychology, and policy; "Correlation does not imply causation" is a common theme for much of the epidemiological literature. Foundations of modern biology There are five unifying principles Sociology (from Latin: socius "companion" and the suffix -ology "the study of" from Greek λόγος lógos "knowledge" Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and Statistics is a mathematical science pertaining to the collection analysis interpretation or explanation and presentation of Data. Anthropology (/ˌænθɹəˈpɒlədʒi/ from Greek grc ἄνθρωπος anthrōpos, "human" -λογία -logia) is the study of Psychology (from Greek grc ψῡχή psȳkhē, "breath life soul" and grc -λογία -logia) is an Academic and A policy is a deliberate plan of action to guide decisions and achieve rational outcome(s Correlation does not imply causation is a phrase used in the Sciences and Statistics to emphasize that Correlation between two variables does not imply For epidemiologists, the key is in the term inference. Inference is the act or process of deriving a Conclusion based solely on what one already knows Epidemiologists use gathered data and a broad range of biomedical and psychosocial theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal, and about exactly how they are causal. Epidemiologists Rothman and Greenland emphasize that the "one cause - one effect" understanding is a simplistic mis-belief. Most outcomes — whether disease or death — are caused by a chain or web consisting of many component causes.

Bradford-Hill criteria

In 1965 Austin Bradford Hill detailed criteria for assessing evidence of causation. Sir Austin Bradford Hill FRS ( July 8, 1897 - April 18, 1991) English Epidemiologist and Statistician ^[11] These guidelines are sometimes referred to as the Bradford-Hill criteria, but this makes it seem like it is some sort of checklist. For example, Phillips and Goodman (2004) note that they are often taught or referenced as a checklist for assessing causality, despite this not being Hill's intention ^[12]. Hill himself said "None of my nine viewpoints can bring indisputable evidence for or against the cause-and-effect hypothesis and none can be required sine qua non"^[11].

1. Strength: A small association does not mean that there is not a causal effect. ^[11]
2. Consistency: Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect. ^[11]
3. Specificity: Causation is likely if a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship. ^[11]
4. Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay). ^[11]
5. Biological gradient: Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence. ^[11]
6. Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge). ^[11]
7. Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. However, Hill noted that ". . . lack of such [laboratory] evidence cannot nullify the epidemiological affect on associations" ^[11].
8. Experiment: "Occasionally it is possible to appeal to experimental evidence" ^[11].
9. Analogy: The effect of similar factors may be considered^[11].

Legal interpretation

Epidemiological studies can only go to prove that an agent could have caused, but not that it did cause, an effect in any particular case:

"Epidemiology is concerned with the incidence of disease in populations and does not address the question of the cause of an individual’s disease. An Epidemiological study is a statistical study on human populations which attempts to link human health effects to a specified cause Incidence is a measure of the risk of developing some new condition within a specified period of time This question, sometimes referred to as specific causation, is beyond the domain of the science of epidemiology. Epidemiology has its limits at the point where an inference is made that the relationship between an agent and a disease is causal (general causation) and where the magnitude of excess risk attributed to the agent has been determined; that is, epidemiology addresses whether an agent can cause a disease, not whether an agent did cause a specific plaintiff’s disease. "^[13]

In United States law, epidemiology alone cannot prove that a causal association does not exist in general. Conversely, it can be (and is in some circumstances) taken by US courts, in an individual case, to justify an inference that a causal association does exist, based upon a balance of probability. Probability is the likelihood or chance that something is the case or will happen

Advocacy

As a public health discipline, epidemiologic evidence is often used to advocate both personal measures like diet change and corporate measures like removal of junk food advertising, with study findings disseminated to the general public in order to help people to make informed decisions about their health. Public health is "the science and art of preventing disease prolonging life and promoting health through the organised efforts and informed choices of society organisations Advocacy Advocacy is the pursuit of influencing outcomes –including public-policy and resource allocation decisions within political economic and social systems Junk food is a broad term that is used to describe a variety of Food products from Sweets that contain little to no Nutritional value to products used in Often the uncertainties about these findings are not communicated well; news articles often prominently report the latest result of one study with little mention of its limitations, caveats, or context. Epidemiological tools have proved effective in establishing major causes of diseases like cholera and lung cancer but have had problems with more subtle health issues, and several recent epidemiological results on medical treatments (for example, on the effects of hormone replacement therapy) have been refuted by later randomized controlled trials. Cholera, sometimes known as Asiatic cholera or epidemic cholera, is an infectious Gastroenteritis caused by the Bacterium Lung cancer is a Disease of uncontrolled Cell growth in tissues of the Lung. Hormone replacement therapy may refer to Hormone replacement therapy (menopause Hormone replacement therapy (female-to-male A randomized controlled trial (RCT is a type of scientific Experiment most commonly used in testing the Efficacy or Effectiveness of Healthcare ^[14]

Population-based health management

Epidemiological practice and the results of epidemiological analysis make a significant contribution to emerging population-based health management frameworks.

Population-based health management encompasses the ability to:

• assess the health states and health needs of a target population;
• implement and evaluate interventions that are designed to improve the health of that population; and
• efficiently and effectively provide care for members of that population in a way that is consistent with the community’s cultural, policy and health resource values.

Modern population-based health management is complex, requiring a multiple set of skills (medical, political, technological, mathematical etc. ) of which epidemiological practice and analysis is a core component, that is unified with management science to provide efficient and effective health care and health guidance to a population. This task requires the forward looking ability of modern risk management approaches that transform health risk factors, incidence, prevalence and mortality statistics (derived from epidemiological analysis) into management metrics that not only guide how a health system responds to current population health issues, but also how a health system can be managed to better respond to future potential population health issues.

Examples of organizations that use population-based health management that leverage the work and results of epidemiological practice include Canadian Strategy for Cancer Control, Health Canada Tobacco Control Programs, Rick Hansen Foundation, Canadian Tobacco Control Research Initiative. ^[15][16][17]

Each of these organizations use a population-based health management framework called Life at Risk that combines epidemiological quantitative analysis with demographics, health agency operational research and economics to perform:

• Population Life Impacts Simulations: Measurement of the future potential impact of disease upon the population with respect to new disease cases, prevalence, premature death as well as potential years of life lost from disability and death;
• Labour Force Life Impacts Simulations: Measurement of the future potential impact of disease upon the labour force with respect to new disease cases, prevalence, premature death and potential years of life lost from disability and death;
• Economic Impacts of Disease Simulations: Measurement of the future potential impact of disease upon private sector disposable income impacts (wages, corporate profits, private health care costs) and public sector disposable income impacts (personal income tax, corporate income tax, consumption taxes, publicly funded health care costs).

Types of studies

Main article: Study design

Case series

Case-series may refer to the qualititative study of the experience of a single patient, or small group of patients with a similar diagnosis, or to a statistical technique comparing periods during which patients are exposed to some factor with the potential to produce illness with periods when they are unexposed. A study design is an analytic approach to conduct an Epidemiological investigation such as a Clinical trial.

The former type of study is purely descriptive and cannot be used to make inferences about the general population of patients with that disease. These types of studies, in which an astute clinician identifies an unusual feature of a disease or a patient's history, may lead to formulation of a new hypothesis. Using the data from the series, analytic studies could be done to investigate possible causal factors. These can include case control studies or prospective studies. A case control study would involve matching comparable controls without the disease to the cases in the series. A prospective study would involve following the case series over time to evaluate the disease’s natural history. ^[18]

The latter type, more formally described as self-controlled case-series studies, divide individual patient follow-up time into exposed and unexposed periods and use fixed-effects poisson regression processes to compare the incidence rate of a given outcome between exposed and unexposed periods. This technique has been extensively used in the study of adverse reactions to vaccination, and has been shown to provide statistical power comparable to that available in cohort studies.

Case control studies

Case control studies select subjects based on their disease status. The study population is comprised of individuals that are disease positive. The control group should come from the same population that gave rise to the cases. The case control study looks back through time at potential exposures both populations (cases and controls) may have encountered. A 2x2 table is constructed, displaying exposed cases (A), the exposed controls (B), unexposed cases (C) and the unexposed controls(D). The statistic generated to measure association is the odds ratio (OR), which is the ratio of the odds of exposure in the cases (A/C) to the odds of exposure in the controls (B/D). The odds ratio is a measure of Effect size particularly important in Bayesian statistics and Logistic regression. This is equal to (A*D)/(B*C).

. . . . .	Cases high	Controls
Exposed low	A	B
Unexposed	C prevalence	D

If the OR is clearly greater than 1, then the conclusion is "those with the disease are more likely to have been exposed," whereas if it is close to 1 then the exposure and disease are not likely associated. If the OR is far less than one, then this suggests that the exposure is a protective factor in the causation of the disease.

Case control studies are usually faster and more cost effective than cohort studies, but are sensitive to bias (such as recall bias and selection bias). A cohort study or panel study is a form of Longitudinal study used in Medicine and Social science. The main challenge is to identify the appropriate control group; the distribution of exposure among the control group should be representative of the distribution in the population that gave rise to the cases. This can be achieved by drawing a random sample from the original population at risk. This has as a consequence that the control group can contain people with the disease under study when the disease has a high attack rate in a population.

Cohort studies

Cohort studies select subjects based on their exposure status. The study subjects should be at risk of the outcome under investigation at the beginning of the cohort study; this usually means that they should be disease free when the cohort study starts. The cohort is followed through time to assess their later outcome status. An example of a cohort study would be the investigation of a cohort of smokers and non-smokers over time to estimate the incidence of lung cancer. The same 2x2 table is constructed as with the case control study. However, the point estimate generated is the Relative Risk (RR) [What is Relative Risk? How is it measured? How can values be interpreted? Link to statistical analysis? Explanation needed], which is the incidence of disease in the exposed group (A/A+B) over the incidence in the unexposed (C/C+D).

. . . . .	Case	Non case	Total
Exposed	A	B	(A+B)
Unexposed	C	D	(C+D)

As with the OR, a RR greater than 1 shows association, where the conclusion can be read "those with the exposure were more likely to develop disease. "

Prospective studies have many benefits over case control studies. The RR is a more powerful effect measure than the OR, as the OR is just an estimation of the RR, since true incidence cannot be calculated in a case control study where subjects are selected based on disease status. Temporality can be established in a prospective study, and confounders are more easily controlled for. However, they are more costly, and there is a greater chance of losing subjects to follow-up based on the long time period over which the cohort is followed.

Outbreak investigation

For information on investigation of infectious disease outbreaks, please see outbreak investigation. An infectious disease is a clinically evident Disease resulting from the presence of Pathogenic microbial agents including Pathogenic viruses Pathogenic Outbreak is a classification used in Epidemiology to describe a small localized group of people or organisms infected with a disease

Validity: precision and bias

Random error

Random error is the result of fluctuations around a true value because of sampling variability. Random error is just that: random. It can occur during data collection, coding, transfer, or analysis. Examples of random error include: poorly worded questions, a misunderstanding in interpreting an individual answer from a particular respondent, or a typographical error during coding. Random error affects measurement in a transient, inconsistent manner and it is impossible to correct for random error.

There is random error in all sampling procedures. This is called sampling error. In Statistics, sampling error or estimation error is the error caused by observing a sample instead of the whole population

Precision in epidemiological variables is a measure of random error. Precision is also inversely related to random error, so that to reduce random error is to increase precision. Confidence intervals are computed to demonstrate the precision of relative risk estimates. The narrower the confidence interval, the more precise the relative risk estimate.

There are two basic ways to reduce random error in an epidemiological study. An Epidemiological study is a statistical study on human populations which attempts to link human health effects to a specified cause The first is to increase the sample size of the study. In other words, add more subjects to your study. The second is to reduce the variability in measurement in the study. This might be accomplished by using a more accurate measuring device or by increasing the number of measurements.

Note, that if sample size or number of measurements are increased, or a more precise measuring tool is purchased, the costs of the study are usually increased. There is usually an uneasy balance between the need for adequate precision and the practical issue of study cost.

Systematic error

A systematic error or bias occurs when there is a difference between the true value (in the population) and the observed value (in the study) from any cause other than sampling variability. An example of systematic error is if, unbeknown to you, the pulse oximeter you are using is set incorrectly and adds two points to the true value each time a measurement is taken. A pulse oximeter is a Medical device that indirectly measures the oxygen saturation of a patient's Blood (as opposed to measuring oxygen saturation directly through Because the error happens in every instance, it is systematic. Conclusions you draw based on that data will still be incorrect. But the error can be reproduced in the future (eg, by using the same mis-set instrument).

A mistake in coding that affects *all* responses for that particular question is another example of a systematic error.

The validity of a study is dependent on the degree of systematic error. Validity is usually separated into two components:

• Internal validity is dependent on the amount of error in measurements, including exposure, disease, and the associations between these variables. Internal validity is the validity of (causal inferences in scientific studies usually based on experiments as experimental validity. Good internal validity implies a lack of error in measurement and suggests that inferences may be drawn at least as they pertain to the subjects under study.

• External validity pertains to the process of generalizing the findings of the study to the population from which the sample was drawn (or even beyond that population to a more universal statement). External validity is the validity of generalized (causal inferences in scientific studies usually based on experiments as experimental validity. This requires an understanding of which conditions are relevant (or irrelevant) to the generalization. Internal validity is clearly a prerequisite for external validity.

Selection bias

Selection bias is one of three types of bias that threatens the internal validity of a study. Selection bias is a distortion of evidence or data that arises from the way that the data are collected Selection bias is an inaccurate measure of effect which results from a systematic difference in the relation between exposure and disease between those who are in the study and those who should be in the study.

If one or more of the sampled groups does not accurately represent the population they are intended to represent, then the results of that comparison may be misleading.

Selection bias can produce either an overestimation or underestimation of the effect measure. It can also produce an effect when none actually exists.

An example of selection bias is volunteer bias. Volunteers may not be representative of the true population. They may exhibit exposures or outcomes which may differ from nonvolunteers (eg volunteers tend to be healthier or they may seek out the study because they already have a problem with the disease being studied and want free treatment).

Another type of selection bias is caused by non-respondents. For example, women who have been subjected to politically motivated sexual assault may be more fearful of participating in a survey measuring incidents of mass rape than non-victims, leading researchers to underestimate the number of rapes.

To reduce selection bias, you should develop explicit (objective) definitions of exposure and/or disease. You should strive for high participation rates. Have a large sample size and randomly select the respondents so that you have a better chance of truly representing the population.

Journals

A ranked list of journals:^[19]

General journals

• American Journal of Epidemiology
• Epidemiologic Reviews
• Epidemiology
• International Journal of Epidemiology
• Annals of Epidemiology
• Journal of Epidemiology and Community Health
• European Journal of Epidemiology
• Emerging Themes in Epidemiology
• Epidemiologic Perspectives and Innovations
• Eurosurveillance

Specialty journals

• Cancer Epidemiology Biomarkers and Prevention
• Genetic Epidemiology
• Journal of Clinical Epidemiology
• Paediatric Perinatal Epidemiology
• Epidemiology and Infection
• Pharmacoepidemiology and Drug Safety

Areas

By physiology/disease

• Infectious disease epidemiology
• Cardiovascular disease epidemiology
• Cancer epidemiology
• Neuroepidemiology
• Epidemiology of Aging
• Oral/Dental epidemiology
• Reproductive epidemiology
• Obesity/diabetes epidemiology
• Renal epidemiology
• Injury epidemiology
• Psychiatric epidemiology
• Veterinary epidemiology
• Epidemiology of zoonosis
• Respiratory Epidemiology
• Pediatric Epidemiology
• Quantitative parasitology

By methodological approach

• Environmental epidemiology
• Economic epidemiology
• Clinical epidemiology
• Conflict epidemiology
• Genetic epidemiology
• Molecular epidemiology
• Nutritional epidemiology
• Social epidemiology
• Lifecourse epidemiology
• Epi methods development / Biostatistics
• Meta-analysis
• Spatial epidemiology
• Tele-epidemiology
• Biomarker epidemiology
• Pharmacoepidemiology
• Primary care epidemiology
• Infection control and hospital epidemiology
• Public Health practice epidemiology
• Surveillance epidemiology (Clinical surveillance)
• Disease Informatics

See also

Footnotes

Sources

• Clayton, David and Michael Hills (1993) Statistical Models in Epidemiology Oxford University Press. David George Clayton, born 13th June 1944 is a distinguished British Statistician and Epidemiologist. ISBN 0-19-852221-5

A thorough introduction to the statistical analysis of epidemiological data, focussing on survival rates - their estimation, analysis and comparison.

• Last JM (2001). "A dictionary of epidemiology", 4th edn, Oxford: Oxford University Press.
• Morabia, Alfredo. ed. (2004) A History of Epidemiologic Methods and Concepts. Basel, Birkhauser Verlag. Part I.
• Smetanin P. , Kobak P. , Moyer C. , Maley O (2005) “The Risk Management of Tobacco Control Research Policy Programs” The World Conference on Tobacco OR Health Conference, July 12–15, 2006 in Washington DC.
• Szklo MM & Nieto FJ (2002). "Epidemiology: beyond the basics", Aspen Publishers, Inc.

External links

• The Health Protection Agency
• The Collection of Biostatistics Research Archive
• Statistical Applications in Genetics and Molecular Biology
• The International Journal of Biostatistics
• Analytical Epidemiology
• BMJ - Epidemiology for the Uninitiated' (fourth edition), D. Coggon, PHD, DM, FRCP, FFOM, Geoffrey Rose DM, DSC, FRCP, FFPHM, DJP Barker, PHD, MD, FRCP, FFPHM, FRCOG, British Medical Journal
• Epidem.com - Epidemiology (peer reviewed scientific journal that publishes original research on epidemiologic topics)
• NIH.gov - 'Epidemiology' (textbook chapter), Philip S. Brachman, Medical Microbiology (fourth edition), US National Center for Biotechnology Information

• UTMB.edu - 'Epidemiology' (plain format chapter), Philip S. Medical microbiology is a branch of Microbiology which deals with the study of microorganisms including bacteria viruses fungi and parasites which are of medical importance The National Center for Biotechnology Information ( NCBI) is part of the United States National Library of Medicine (NLM a branch of the National Institutes Brachman, Medical Microbiology

• Monash Virtual Laboratory - Simulations of epidemic spread across a landscape

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