In cohort type studies one identifies individuals who do not have the outcome it suggests an association between smoking and risk of developing a heart attack. . The descriptive epidemiology indicated that almost all of the cases belonged. Typically, descriptive studies are most useful relations and etiologic factors. Aug 30, Epidemiological analytical studies analyse relationships between the case- control study; the use of this type of study has been repeatedly.
Shortly thereafter, the Food and Drug Administration issued an advisory to the public about green onions and risk of hepatitis A.
This action was in direct response to the convincing results of the analytic epidemiology, which compared the exposure history of case-patients with that of an appropriate comparison group.
When investigators find that persons with a particular characteristic are more likely than those without the characteristic to contract a disease, the characteristic is said to be associated with the disease.
The characteristic may be a: Demographic factor such as age, race, or sex; Constitutional factor such as blood group or immune status; Behavior or act such as smoking or having eaten salsa; or Circumstance such as living near a toxic waste site. Identifying factors associated with disease help health officials appropriately target public health prevention and control activities. It also guides additional research into the causes of disease.
Thus, analytic epidemiology is concerned with the search for causes and effects, or the why and the how. Epidemiologists use analytic epidemiology to quantify the association between exposures and outcomes and to test hypotheses about causal relationships.
It has been said that epidemiology by itself can never prove that a particular exposure caused a particular outcome. Often, however, epidemiology provides sufficient evidence to take appropriate control and prevention measures.
Epidemiologic studies fall into two categories: Experimental studies In an experimental study, the investigator determines through a controlled process the exposure for each individual clinical trial or community community trialand then tracks the individuals or communities over time to detect the effects of the exposure. For example, in a clinical trial of a new vaccine, the investigator may randomly assign some of the participants to receive the new vaccine, while others receive a placebo shot.
The investigator then tracks all participants, observes who gets the disease that the new vaccine is intended to prevent, and compares the two groups new vaccine vs. Similarly, in a trial to prevent onset of diabetes among high-risk individuals, investigators randomly assigned enrollees to one of three groups — placebo, an anti-diabetes drug, or lifestyle intervention.
At the end of the follow-up period, investigators found the lowest incidence of diabetes in the lifestyle intervention group, the next lowest in the anti-diabetic drug group, and the highest in the placebo group. John Snow's studies of cholera in London were observational studies. The two most common types of observational studies are cohort studies and case-control studies; a third type is cross-sectional studies.
A cohort study is similar in concept to the experimental study. In a cohort study the epidemiologist records whether each study participant is exposed or not, and then tracks the participants to see if they develop the disease of interest. In general, environmental epidemiology is an observational rather than an experimental science; scientific deductions are drawn from patterns of occurrence.
- Origins of Epidemiology
- Analytical studies
- Prospective Cohort Studies
Its principal aim is to identify risk factors that can be averted or reduced so as to prevent or reduce the risk of future disease and promote public health. Types of Studies in Environmental Epidemiology Environmental-epidemiologic studies can be classified broadly into 2 categories that are not mutually exclusive: Typically, descriptive studies are most useful for generating hypotheses and analytic studies most useful for testing hypotheses, though each type of study can be used for both purposes.
Whether a study is hypothesis-testing or hypothesis-generating depends more on the sequence of past studies and the present state of knowledge i. Recent innovations in descriptive studies sometimes permit refined assessments of dose-response relations and etiologic factors.
Descriptive Studies Descriptive studies include case reports, surveillance systems, ecologic studies, and cluster studies WHO, Case Reports A case report is a descriptive study of a single individual or small group in which the study of an association between an observed effect and a specific environmental exposure is based on detailed clinical evaluations and histories of the individual s. These reports require few financial or personnel resources other than those of clinical medicine, and they may indicate whether additional study of a larger group of persons with similar health problems and exposures should be undertaken.
However, the value of case reports is often limited because they lack a context of the disease in unexposed persons, variables such as time and dose of exposure are generally not known, and controls are absent. They are most likely to be useful when the disease is uncommon and when it is caused exclusively or almost exclusively by a single kind of exposure. In spite of these limitations, many known human environmental toxicants e. Public-health agencies must often investigate clusters of cases that are reported to them by private physicians and others.
While case reports may not lead to identification of new causes of disease, they are more likely to point to specific hypotheses and to biologically meaningful associations if either the disease or the exposure is relatively rare. Surveillance Systems These systems provide broad-scale information on specified populations for which epidemiologic analyses can be conducted.
Surveillance systems are generally designed to attain complete or nearly complete coverage of every identified instance of certain defined conditions in a defined population. Thus, they can be used to estimate the background incidence and prevalence of adverse effects, and trends can be analyzed across time and between populations or geographic areas. Surveillance systems can identify increases or decreases in the occurrence of deaths from specific diseases and thus suggest or test hypotheses related to environmental exposures.
Principles of Epidemiology | Lesson 1 - Section 7
For example, observations of a decline in age-adjusted stomach-cancer rates over time in the United States have stimulated the development of hypotheses about changes in dietary habits in the population as a whole, as well as about changes in the use of food preservatives and refrigeration Howson et al.
Similarly, after postmenopausal estrogen use fell in the United States, rates of endometrial carcinoma declined in women over age 65, lending support to an inference drawn from case-control studies that postmenopausal estrogen use increased the risk of endometrial cancer Austin and Roe, In another instance, surveillance data from the National Center for Health Statistics suggested that a fall in blood-lead levels in US children was linked to a drop in gasoline-lead levels Annest et al.
As public-health agencies have expanded the scope of surveillance systems see chapter 5it has become feasible to study the relationships between disease patterns and variations in environmental factors. Surveillance systems are expected to become increasingly common because the quality of their data is rising, statistical methods are improving, and costs are declining. The Agency for Toxic Substances and Disease Registry ATSDR has devised several surveillance systems to monitor the health of persons believed to have incurred exposure to such substances as trichloroethylene and dioxin.
No results are yet available from those systems. If these exposure registries are to produce valuable results, they will need to include sufficient numbers of persons over a long enough period for diseases of interest to manifest themselves in numbers sufficient to demonstrate that some problem exists or that the problem is unlikely to exist and be large enough to cause serious concern.
Ecologic Studies Ecologic studies explore the statistical connection between disease and estimated exposures in population groups rather than individuals.
They combine data from vital records, hospital discharges, or disease registries with grouped data or estimates of exposure, such as factory emissions in a given geographic area, proximity to waste sites, or air or water pollution levels.
Observed associations may provide support for further investigations. Ecologic studies suffer from serious weaknesses: Although some population groups in ecologic studies may appear similar to "cohorts" see belowthey lack the individual data that permit their analysis in a cohort study.
The "ecologic fallacy" refers to drawing inferences incorrectly from data on groups or about individuals in the groups. Several advances have facilitated an increase in the number of ecologic studies, including the development of surveillance systems, improved environmental-exposure databases e. The value of ecologic studies may be strengthened as methods for estimating exposure are improved. Where valid proxies for gradations of exposure and relevant confounding variables can be devised, the ecologic fallacy may be reduced or overcome.
Ecologic investigations have provided important clues about causal associations even though these studies can be difficult to interpret. For example, fluoride was first found to prevent dental caries on the basis of observed correlations between geographic variations in natural levels of fluoride and rates of tooth decay Dean et al.
Analytical Studies - WikiLectures
Similarly, rates of cardiovascular disease and cancer among immigrants have been correlated with those of their newly acquired compatriots, suggesting that changes in dietary and other factors are involved.
Further refinements in the parameters of interest in ecologic studies might permit these studies to generate more-precise indications of associations between risk factors and disease Greenland, Studies of health problems in relation to fixed sources of environmental exposure have often relied on either labor-intensive techniques, such as personal interviews, or much more general classifications, such as ZIP code or town of residence.
The latter approach has the obvious problems of errors in classification of actual residence and of including people in the exposed category who live far from the site and have little opportunity for exposure. This difficulty has been partially overcome by including better geographic-location information as part of state and federal lists of potential sources of environmental exposure e.
This additional information, along with the availability of improved mapping software, has greatly improved our ability to link health data, such as cancer incidence with residence near a source of environmental exposure. With good geographic coding, disease cases and controls can be readily and quickly located in relation to an environmental source so that various measures of distance and direction can be studied.
Data on the number and characteristics of people living in the area can also be obtained from census data. Cluster Studies A cluster study is a descriptive study of the population in a geographic area, occupational setting, or other small group in which the rate of a specific adverse effect is much higher than expected.
Further, the group is often defined after the fact; that is, the "cluster" comes to attention, and the group is then defined so as to include it. Thus, clusters usually have the drawbacks of small samples. Cluster studies suffer from a major tautology: For example, a reported cluster of cardiac birth defects that occur near a hazardous-waste site may be "tested" by comparing the measured rates of these defects in the same given geographic area with those from outside the same area.
This is a highly unreliable approach methodologically and statistically, as the sample being studied has not been randomly selected.
Nonetheless, the approach can be useful when the relative risk is extremely high, and it can be useful in developing hypotheses for study with other data. Many occupational hazards were first identified because clusters of disease were detected in specific workplaces, and other environmental diseases may also be ascertainable through cluster analysis.
Analytic Studies In contrast to descriptive studies, analytic studies are based on more individually detailed data from individuals that can be used to control for confounding, and they are usually more costly and labor-intensive. Information from medical records, clinical or laboratory investigations, questionnaire results, or direct measures or estimates of exposures may allow analytic studies to explore hypotheses about suspected causes of disease or identify and measure risk factors that increase the chance that a given disease will occur.
Lesson 1: Introduction to Epidemiology
Analytic studies may also be a source of additional specific hypotheses, often leading to a sequence of studies, the more recent being designed to attempt to refute hypotheses raised by earlier studies. The classic designs of analytical studies are case-control and cohort studies. In addition, 2 "hybrid" designs—nested case-control studies and case-cohort studies—can be based on identified cohorts.
Case-Control Studies Case-control studies compare exposures of individuals who have a specific adverse effect or disease with exposures of controls who do not have the effect or disease; controls generally come from the same population from which the cases were derived.
There is an extensive literature on the design of case-control studies, including selection of controls, correction for confounding, statistical methods for analysis, and presentation of measures of effect, usually the odds ratio Schlesselman, These studies generally depend on the collection of retrospective data.
They may suffer from recall bias, i. However, it is often possible in a case-control study to collect histories of exposure to many different factors and control for confounding more efficiently than in a large cohort study, where the costs of collecting substantial exposure data from all the members of the cohort may be prohibitive.
It is likely that case-control studies will be conducted with increasing frequency as new ways of characterizing exposure through the use of biologic markers are developed see chapter 3mirroring the development that has occurred in the last 2 decades in other areas of epidemiology. Cohort Studies These studies identify a group of persons called a cohort, or sometimes several cohorts with differing kinds of the exposures of interest.
Sometimes, a control group has zero exposure. The cohort study evaluates associations between the exposure s and 1 or more health outcomes in the cohort s.
In a cohort study, individuals with differing exposures to a suspected risk factor are identified and then observed for the occurrence of certain health effects over some period, commonly years rather than weeks or months. By properly selecting groups of exposed people we can study relatively rare exposures. Detailed information on confounding factors can be collected allowing control for them either in the analysis or design.
The disadvantage of cohort studies is that they can take long time and generally are expensive. For rare diseases, the number of subjects that need to be studied is often so large since there is a huge majority likely not show the diseaseas to make cohort study impractical or unfeasible.
Case-control studies compare exposures in disease cases vs. These studies can be used to evaluate many different exposures and are relatively quick to be conducted.