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epidemiology - RESEARCH DESIGNS IN EPIDEMIOLOGY I
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RESEARCH DESIGNS IN EPIDEMIOLOGY I
One of the main tasks of epidemiology is to identify the forces and factors which influence the onset and course of disease and compromise the health of individuals and populations.
When considering disease we refer to causes or risk factors ; when considering health we refer to determinants. Risk factors tend to be specific (eg. smoking, plaque) while determinants tend to be broader and more general (eg. socioeconomic status, education).
This lecture is the first of three that describes the research approaches epidemiologists use to identify these causes of disease and determinants of health. The lectures are also concerned with the strengths and weaknesses of the designs used in epidemiology in terms of establishing causal relationships.
What do we mean by cause?
When we think of causal relationships we often think in terms of what is in fact an extreme situation. That is where an event B always and invariably follows from another event A. This type of causation is often observed in the physical sciences.
Fixed mass of gas at Increase pressure Decrease volume
In this type of causation, the causal agent is sufficient and invariably produces the outcome.
Causal relationships with respect to health and disease are rarely this simple. For example, while tuberculosis is caused by infection with mycobacterium tuberculosis, infection does not invariably lead to the disease. Only a small proportion of those infected develop tuberculosis. Other factors are clearly involved.
Infection with mycobacterium tuberculosis
Poor nutrition Tuberculosis
In this case the tubercle bacillus is a necessary but not a sufficient cause of the disease. It is necessary because the outcome never happens in the absence of the causal factor, but it alone is not sufficient to cause disease.
A more complex situation arises when consider smoking and lung cancer. Even though we consider that smoking is a cause of lung cancer, it is neither sufficient nor necessary to cause the disease. Some people who smoke do not get the disease while some people who do not smoke do get the disease. Here, smoking increases the frequency or probability of the disease, but the outcome does not always result and the outcome can occur in its absence.
Causal factors and preventive factors
When considering health issues a causal factor is considered to be a factor that increases the frequency of a disease. The opposite of a causal factor is a preventive factor; its operation decreases the frequency of a disease.
In epidemiology we tend to talk about risk factors rather then causes. Defined simply, risk is the probability that an individual will acquire a given disease in a defined period of time.
Concepts of risk
RISK FACTOR: an attribute that increases the probability of a disease and
is causally related to onset
RISK INDICATOR: an attribute that is associated with disease ...
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