Epidemiological Study Of Disease

What is an 'observational study'?

Observational studies are the method of choice of epidemiology and, in many respects, one of the features that characterizes epidemiology. As one could imagine, it's not possible to cause specific diseases in the population, so the epidemiologist instead takes advantage of naturally occurring situations or observational situations in which to observe the occurrence of disease. An example of that might be things that are done, such as public health methods, or interventions that are taken to control diseases in the population. For example, when fluoridation of water was applied to communities to evaluate the effects of water fluoridation. In a sense, this was kind of a naturally occurring situation that could be observed by epidemiologists. Another example has to do with the many public health interventions that we have to protect the safety of the public, such as the use of lap belts to prevent injury during automobile accidents, and lowering the speed limit, and so forth. These are all naturally occurring situations, or situations that were introduced by statute, and they have great implications for health of the population and could be studied from the epidemiologic point of view.

What do epidemiologists mean by the term 'natural experiments'?

Well, a natural experiment is one that takes advantage of a naturally occurring situation. One of the best examples of that is the work, or was the work, of John Snow during the 1800's. Snow was interested in trying to determine the cause of a major outbreak of cholera that was occurring in the Soho district in London during that time. And what he found out was that this particular region of Soho was served by two different water companies, and then he could try to show the correlation between the source of the water and contamination from the river Thames that supplied the water companies. It turned out the reason why it was a natural experiment, was that one of the companies changed the source of the water to a purer source from the Thames river, and then he was able to show that the people who had the purer source of water had lower amounts of cholera than those who had the existing source, or who maintained the same water source.

What is a 'cohort study'?

A cohort study is a specific study used in epidemiology that first identifies individuals on the basis of their exposure to a disease causing factor, such as exposure to a toxic chemical or other factor that may be implicated in a disease. It begins by taking a group of people who are free from the disease of interest. For example, the epidemiologist may hypothesize that exposure to a specific chemical may be related to a certain form of lung disease. It's important to then rule out or to eliminate all individuals who have any evidence that is being studied. The epidemiologist then follows the individuals over a period of time and observes new occurrences of the disease. It is called a cohort because a cohort of individuals refers to a group of individuals who are similar with respect to exposure to a certain disease causing factor.

Why do epidemiologists use cohort studies?

Some of the advantages of a cohort study, perhaps the most important one, is the fact that the cohort studies make it possible to directly assess risk of coming down with the condition or developing the condition if one is exposed to the factor. So they are among the most powerful studies designed with respect to studying the ideology of the disease. In addition cohort studies solve the problem of temporality. In other words we want to see the cart follow the horse rather than the reverse of that, the cart after the horse. So in a cohort study we first have the appearance of exposure and then the development of the disease. Finally, cohort studies make it possible to study exposure that are rare to the population such as those that can occur on occupational groups, those who work with toxic chemicals or industrial groups and so forth.

What are the drawbacks of cohort studies?

Well, the drawbacks of cohort studies is that they often require large numbers of subjects in order to observe the outcome, and the occurrence of the outcome. They require long periods of time because of the latency period required for certain types of disease. And, by their very nature, because they take a long period of time, and require a large number of subjects; they are expensive to operate.

What is a 'case-control' study?

A case control study is a type of etiologic study that is epidemiology study, that involves the comparison of two groups. The first group is called the cases and the cases are a group of individuals who have a specific disease that we are interested in studying. The controls refer to a group of people who do not have the condition that we are investigating, but in many ways are similar to the case group. In a 'case-control' study, we use what is known as a retrospective approach. The retrospective approach means that we investigate the history of the individuals to find out whether or not they have had an exposure to a specific etiologic agent or factor that we would like to try to relate to the disease or outcome of interest.

Why do epidemiologists use case-controlled studies?

The reasons why epidemiologists use case-controlled studies is that they often provide a rapid answer to pressing problems. For example, when there is an outbreak of food-borne illness, it is not unusual to conduct a case-controlled study to quickly identify the agents, or suspected agents, that were involved in the outbreak. Case-controlled studies enable us to get a quick answer to pressing and mysterious problems that are occurring. Another advantage of case-controlled studies is that they are rapid. They are less expensive, and they also permit the study of diseases that are uncommon in the population, so we can identify people who have a rather unusual disease, include them in the case group, and then select controls that we will use in the case-control group for the case-controlled study.

What are the drawbacks of case-control studies?

There are some drawbacks to the case-control study. One of those stems from the use of the retrospective approach to analyze exposures or to try to identify exposures to a factor. We are never really certain at what time the exposure occurred. We are looking retrospectively into the exposure to the disease, but our information about exposure is more fallible, depending on the source of the information that we use to gather the exposure. For example, if we ask the individuals in the study, the participants about their exposure some of them might have forgotten or been unaware of their past exposure.

What is an 'ecologic study'?

An ecologic study requires somewhat of a change of perspective to think about it. It is a study that analyzes the occurrence of disease or investigates the occurrence of disease among groups; where the groups are the unit of observation. In most of the studies that we consider, the individual is the unit of observation. In an ecologic study, the group may be the unit of of observation. For example, if we study the occurrence of help problems related to air pollution, our group might be the county. We could look at the occurrence at the county level of specific lung diseases, and then identify pollution levels at the county level, and try to show, demonstrate or assess the association between exposure and the outcome. Another example of a ecologic study would be if we compare different countries with respect to their dietary practices. For example, the amount of fat that they consume in their diets and the occurrence of breast cancer mortality. In this example, the unit of analysis would be the country.

Why do epidemiologists use ecological studies?

Ecological studies are also a way to gather information quickly in order to achieve answers rapidly. They are inexpensive to conduct, however the results that they give us are often unsatisfactory because we do not have precise information about exposure and disease outcome. Another difficulty with ecology studies is that they are prone to what is known as the ecologic fallacy. That is the inability to generalize information that we gathered at the group level to specific individuals.

What is a 'cross-sectional' study?

A cross-sectional study is a type of study that is referenced about a single point in time. In other words, information about the exposure factor, and the outcome variable, are collected at the same point in time. Now, this may seem as a somewhat puzzling statement, but it's used very commonly, and will become clear when I give an example. An example would be a survey that is conducted of the population. It is possible to conduct a survey in which information about exposure is collected, and information about disease outcomes are collected at the same time. This is an example of the cross-sectional approach.

Why do epidemiologists use cross-sectional studies?

Cross-sectional studies are used in epidemiology primarily for the purpose of developing interesting hypotheses and showing trends in the distribution of diseases in the population that can be followed up in other types of studies. A cross-sectional study is a type of descriptive epidemiologic study that can lead the way to additional analytic studies such as a case controlled study or a cohort study.

What is an 'occupational epidemiological study'?

An occupational epidemiologic study is one that applies the methods of epidemiology to specific occupational health problems. The world of work and occupations are filled with many questions of interest to epidemiology, particularly in the aetiology of chronic diseases such as cancer, and heart disease. Occupational epidemiology simply uses the full range of techniques that are available within epidemiology to study these types of problems

Why do epidemiologists use occupational studies?

Occupational health problems are a significant aspect of environmental health, and many occupations carry risks of specific diseases. For example, the construction industry has very high rates of mortality. Other types of industrial processes carry risks of morbidity and mortality. It is very important for our society to try to reduce morbidity and mortality from these occupational causes, and for this reason, epidemiologists are interested in focusing on these problems.