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Basic epidemiology concepts

What is epidemiology?

Epidemiology is the study of the factors affecting the health and illness of populations and how disease is distributed. It serves as the foundation for public health and preventive medicine. Epidemiology is based on observation, not experiments.

What are the three main types of epidemiological studies?

The cohort study starts with a defined group of individuals (the cohort) that have been or may be exposed to a potential specific factor (for example, occupational radiation exposure) at different levels that may cause a given disease. It is the strongest type of epidemiological study.

The case-control study starts with a group of persons recently diagnosed with a specific disease (the “cases”) and are matched (for example, by age or sex) with a control group of persons without the disease (the “controls”). The purpose is to determine if the level of exposure to a specific factor(s) differs in the two groups (the cases and the controls), and to establish a relationship between the disease and the factor. This type of study is prone to more biases than the cohort study; thus, cohort studies are regarded as more reliable than case-control studies.

The descriptive ecological study compares the observed occurrence of a specific disease within a defined population, time and geographical area to the expected occurrence of the disease, based on a stable population (for example, the general population of Ontario or Canada). This type of study can identify trends over time or within groups and are useful monitoring tools to identify high and low rates of disease in a population. This is the weakest type of epidemiological study since it operates at a group level, not an individual level.

What are the advantages or disadvantages to using one type of study over another?

Table – The advantages or disadvantages to using one type of study over another
Study type Advantage Disadvantage
Cohort Effective in studying rare exposures Expensive, time-consuming and the most logistically difficult
Exposure precedes the health outcome Requires a large number of subjects
Can evaluate multiple health outcomes for a given exposure Members of the cohort need to be in contact with the people conducting the study for long periods of time
Less vulnerable to bias Cohort members may not continue to participate (for various reasons) in the study after several years
Case-control Detailed histories of exposure and other information can be collected relatively quickly and easily Vulnerable to selection and recall biases
Useful to follow rare health outcomes Difficult to collect accurate information
Number of subjects can be small Difficult to select appropriate control group and matching on variables
Relatively inexpensive  
Can identify more than one risk factor  
Descriptive ecological Relatively simple, easy and inexpensive Precision of the statistics may be limited due to the small numbers of observed and expected cases or deaths
Although not exposure specific, can evaluate a wide range of exposures Results are averaged over a group, not on an individual level
Often rely on pre-existing data Does not consider multiple risk factors (i.e., smoking, diet, exercise, etc.)

What makes a study valid?

A study is considered valid when the following three alternative explanations have been eliminated:

  • Bias – An error in the design of a study that can influence the results or conclusions of the study. This type of error could mask the true association between the exposure and the disease.

  • Confounding – A confounding factor is one that is associated with both the disease and the exposure. For example, when studying the relationship between an exposure (radon) and a disease (lung cancer) the true effect could be masked by a third factor, known as the confounding factor. In this case, it would be tobacco smoking, which is associated with both the exposure (uranium miners tend to be heavy smokers) and the disease (tobacco is the leading cause of lung cancer).
  • Random error – The probability that the observed result is due to chance.

How does the Canadian Nuclear Safety Commission (CNSC) use epidemiology studies to protect nuclear workers and the Canadian public?

Epidemiology studies are a useful tool to establish the current scientific understanding of the health risks related to radiation exposure. The CNSC uses these studies and the international understanding of the risks from radiation to ensure its regulations appropriately protect Canadian workers.

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