epidemiology - Epidemiology as causal inference, Legal interpretation of epidemiologic studies, Epidemiology and advocacy, Types of Studies
The study of the distribution and causes of disease in populations. In the 19th-c, the major causes of death were infections. Study of the occurrence of outbreaks in relation to the social conditions prevailing at the time led to effective measures for their control. For example, epidemics of cholera were traced to polluted water, and of puerperal fever to the contaminated hands of medical attendants. In the 20th-c the emphasis changed with the changing pattern of disease and the increased sophistication of epidemiological methods. Scientific techniques such as cohort studies and case-controlled studies are used to establish the links between environmental and lifestyle factors and disease. Thus, the link between cancer of the lung and smoking was established, and information about the relative occurrence of coronary heart disease in different countries and of cancers of different types obtained. It is likely that advances in this field of activity will assist in the prevention of these and many other complaints.
Epidemiology is the scientific 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. 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.
The acting epidemiologist works on issues ranging from the practical, such as outbreak investigation, environmental exposure, and health promotion, to the theoretical, including the development of statistical, mathematical, philosophical, biological, and psychosocial theory. To this end, epidemiologists employ a range of study designs from the observational to experimental, with the purpose of revealing unbiased relationships between exposures such as nutrition, biological agents, stress, or chemicals to outcomes such as disease, wellness and health indicators. Defining the diseases, drawing disease causal chain / chains, and formulation of health strategy are important aspects of epidemiology.
Epidemiologic studies are generally categorized as descriptive, analytic (aiming to examine associations, commonly hypothesized causal relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions). Some epidemiologists work 'in the field', i.e., in the community, commonly in a public health service, and are often at the forefront of investigating and combating disease outbreaks.
The term 'epidemiologic triangle' is used to describe the intersection of Host, Agent, and Environment in analyzing an outbreak. 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); It is also applied to studies of micro-organisms (microbial epidemiology)
Epidemiology 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 equal causation," is a common theme to much of the epidemiologic literature.
Bradford-Hill guidelines
Austin Bradford Hill outlined a series of 9 guidelines for assessing evidence of causation in 1965:
Strength: A small association does not mean that there is not a causal effect.Legal interpretation of epidemiologic studies
In United States law, epidemiology alone cannot prove that a causal association does not exist in general. Strictly speaking, epidemiology can only go to prove that an agent could have caused but not that, in any particular case, it did cause: "Epidemiology is concerned with the incidence of disease in populations and does not address the question of the cause of an individual’s disease. 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." )
Epidemiology and advocacy
Epidemiology is one of the main resources of public health. In many cases, epidemiologic evidence has to be disseminated to the general public in order to obtain health benefits, and to help people to make informed decisions about their health. For example, consider these two alternative admonishments against smoking:
Smoking has been consistently linked to health problems such as lung cancer and coronary heart disease in several large prospective studies, this link has been deemed causal by a complex
process of induction, consensus, and modeling.The best public health advocates consider the broader context beyond the epidemiology and public health literature to render judgment on a course of action for a population. In this manner they are employing a different analytical framework than the strict scientific method that is more common in scientific epidemiology.
Types of Studies
Case Series
Case-series describe the experience of a single patient or a group of patients with a similar diagnosis. 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. 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. (1987)‚ Epidemiology in Medicine.™ Mayrent, S.L (Ed.), Lippincott, Williams and Wilkins)
Case control studies
Case control studies select subjects based on their disease status. The study population is comprised of individuals that are disease positive, while the controls are disease negative. A 2x2 table is constucted, displaying the individuals that are disease positive and exposure positive (A), disease positive and exposure negative (B), disease negative and exposure positive (C), and disease negative and exposure negative (D). If the OR is greater than 1, then the conclusion is the "those with the disease are more likely to have the exposure," wherease if it is less than 1 the exposure and disease are not associated.
Case control studies are faster and more cost effective than longer prospective studies, but are sensative to bias such as recall bias, and also cannot show that the exposure definitely occurred before the disease.
Prospective studies
Prospective studies, also called cohort studies, select subjects based on their exposure status, and subjects are generally healthy at the beginning of the study. However, the statistic generated is the Relative Risk (RR), which is the incidence of disease in the exposured group (A/A+B) over the incidence in the unexposed (C/C+D).
Prospective studies have many benefits over case control studies. The RR is a more powerful statistic 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.
An online epidemiology discussion forum is available to discuss study designs and analysis methods. Nieto, 2000) Hazard rate Cumulative incidence Prevalence measures Point prevalence
Period prevalence Measures of association Relative measures Risk ratio Rate ratio Odds ratio Hazard ratio Absolute measures Risk/rate/incidence difference Attributable risk Attributable risk in
exposed Percent attributable risk Levin’s attributable risk Other measures Virulence and Infectivity Mortality rate and Morbidity Case fatality Sensitivity (tests) and Specificity (tests)
History of epidemiology
John Graunt, a professional haberdasher and serious amateur scientist, published Natural and Political Observations ... (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.
In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross, Anderson Gray McKendrick and others.
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.
Epidemiology Journals
General Epidemiology 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 InnovationsSpecialty Epidemiology Journals
Cancer Epidemiology Biomarkers and Prevention Genetic Epidemiology Journal of Clinical Epidemiology Paediatric Perinatal Epidemiology Epidemiology and InfectionA ranked list of journals: Impact Factors of leading epidemiology journals
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