The study of disease distributions in the populations and the factors that influence this distribution is called as Epidemiology. In simple words, it is the study of the frequency with which diseases affect different groups of people and the reasons why they occur. Epidemiology has been quite helpful in determining and measuring the health hazards of smoking cigarettes or exposure to materials like asbestos. In this article, you will learn about the types of epidemiological studies and gain information regarding the issues which are to be kept in mind when understanding the results of the various study designs.
Types of Epidemiological Studies
There are two main types of epidemiological studies: experimental studies and observational studies and both of them are divided into several subtypes.
1. Observational Studies
Observational studies are one of the most common types of epidemiological studies. They comprise of simple questioning, medical examinations and routine laboratory tests or X-rays. Below are its four study methods:
- Cross-Section Comparison Studies
Cross-section comparison studies focus on comparing data collected from various smaller groups instead of large groups. These studies can be completed in a very small period of time and usually do not cost that much as well because their target is to obtain observations that are made at one point in a time. However, since these studies only sample data once, they cannot tell the sequence of events that have taken place over a longer period of time. One of the examples of cross-section comparison studies is comparing rate of a type of cancer in one place with that of another.
- Correlation (Ecological) Studies
Ecological studies try to find a correlation between diet and cancer at a population level. Thus, the sample size of these types of studies is quite large. Although these studies try to find a connection between cancer rates of populations and risk factors, representing a shift towards analytical studies, they cannot be considered analytical since they do not provide an outcome.
One of the major advantages of ecological studies is that they can offer strong clues towards a particular direction. On the downside, these types of studies can't prove cause and effect and they require evidence to be gathered from other studies to prove their findings.
- Cohort Studies
Cohort studies are analytical studies which compare populations that are similar to each other but have had different exposure to certain factors like cigarette smoke, asbestos, sun, exercise and diet. These studies try to determine whether the changes in exposure have any effect on the likelihood of getting the disease or not. Cohort studies are performed either using a prospective (over time) approach or a retrospective (historical) approach. These studies are able to provide the scientists with a large amount of data as they give them the chance to follow people over long periods of time.
- Case Control Studies
Case control studies are analytical studies which compare people that have been diagnosed with a disease with people that haven't been diagnosed with it. The diseased people are known as cases while the healthy ones are known as controls. These studies make use of a number of data sources like hospital and medical records and personal interviews of both the cases and the controls. This data is then compared in relation to the exposures that they had in the past to determine the differentiating factors between the two groups. The biggest drawback of these studies is that they heavily rely on the unreliable memories of people.
2. Experimental Studies
Experimental studies are also main types of epidemiological studies that scientists will carry out experiments where they change things in some sets and compare the outcomes.
- In Vitro Studies
Under in Vitro studies, a piece of human or animal cell is usually removed from the body for the experiment. These types of experiments are carried out inside a test tube. Scientists are trying to observe the chain reactions that take place between the cells of the body and the nutrient that has been consumed. These experimental studies can help in identifying which nutrients can help in protecting against cancer and why this happens. In vitro studies are very strictly structured and the scientist is in full control of all the variables. The downside of these studies is that their findings are only applicable at a cellular level.
- Clinical Trials
Clinical trials are experimental studies in which a group of patients is chosen as the subjects. The goal of a clinical trial is to assess a new form of treatment or cure for a disease or to discover a preventive measure for diseases like disability or death. Clinical trials are done by choosing a set of patients having a same level of baseline characteristics. Half of the subjects are given the new treatment while the other half is either given a placebo or the best accepted treatment for their particular condition. It is important to employ blinding during the treatment assignment to prevent any biases.
- Field Trials
Field trials are experimental studies which are slightly different from clinical trials. The difference is that they don't have patients as subjects and are conducted in the field rather than in a clinical environment. Field trials are conducted by visiting the subjects personally while they are at their homes, schools or workplaces. Since field trials are done in the field, they are more expensive than clinical trials and require a large number of subjects too. Due to the high cost, field trials are only conducted for assessing preventives for extremely dangerous or extremely common diseases.