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Chapter 3

Chapter 3 Notes

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Larry Sawchuk

ANTC67: Midterm Notes Chapter 3: Types of Studies Epidemiological studies can be classified as either observational or experimental. Observational Studies Observational studies all nature to take its course: the investigator measures but does not intervene. They include studies that can be called descriptive or analytical: o A descriptive study is limited to a description of the occurrence of a disease in a population and is often the first step in an epidemiological investigation. o An analytical study goes further by analysing relationships between health status and other variables. Apart from the simplest descriptive studies, almost all epidemiological studies are analytical in character. Pure descriptive studies are rare, but descriptive data in reports of health statistics are a useful source of ideas for epidemiological studies. Experimental Studies Experimental or intervention studies involve an active attempt to change a disease determinant J such as an exposure or a behaviour J or the progress of a disease through treatment, and are similar in design to experiments in other sciences. Major experimental study designs include the following: o Randomized controlled trials using patients as subjects (clinical trials) o Field trials in which the participants are healthy people, and o Community trials in which the participants are the communities themselves. In all epidemiological studies it is essential to have a clear definition of a case of the disease being investigated by delineating the symptoms, signs or other characteristics indicating that a person has the disease. In the absence of clear definitions of disease and exposures, it is very difficult to interpret the data from an epidemiological study. Observational Epidemiology Descriptive Studies Pure descriptive studies make no attempt to analyse the links between exposure and effect. They are usually based on mortality statistics and may examine patterns of death by age, sex or ethnicity during specified time periods or in various countries. Ecological Studies Ecological (or correlational) studies are useful for generating hypotheses. In an ecological study, the units of analysis are groups of people of people rather than individuals. Ecological studies can also be done by comparing populations in different places at the same time or, in a time series, by comparing the same population in one place at different times. Time series may reduce some of the socioeconomic confounding that is a potential problem in ecological studies. If the time period in a time series is very short, as it is in daily time series studies confounding is virtually zero as the people in the study serve as their own controls. Although simple to conduct and the attractive, ecological studies are often difficult to interpret since it is seldom possible to examine directly the various potential explanations for findings. Ecological studies usually rely on socioeconomic factors may not be available. One attraction of ecological studies is that data can be used from populations with widely differing characteristics or extracted from different data sources. Ecological Fallacy An ecological fallacy or bias results if inappropriate conclusions are drawn on the basis of ecological data. 1 www.notesolution.com
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