Chapter 3 Notes
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ANTC67: Midterm Notes
Chapter 3: Types of Studies
¾ Epidemiological studies can be classified as either observational or experimental.
¾ 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
o An analytical study goes further by analysing relationships between health status and other variables.
¾ Apart from the simplest descriptive studies, almost all epidemiological studies are analytical in character.
¾ Pure descriptive studies are rare, but descriptive data in reports of health statistics are a useful source of ideas for epidemiological
¾ Experimental or intervention studies involve an active attempt to change a disease determinant t such as an exposure or a behaviour t or
the progress of a disease through treatment, and are similar in design to experiments in other sciences.
¾ 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.
¾ 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.
¾ In the absence of clear definitions of disease and exposures, it is very difficult to interpret the data from an epidemiological study.
¾ Pure descriptive studies make no attempt to analyse the links between exposure and effect.
¾ 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.
¾ 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.
¾ 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.
¾ 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
¾ 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.
¾ One attraction of ecological studies is that data can be used from populations with widely differing characteristics or extracted from
different data sources.
¾ An ecological fallacy or bias results if inappropriate conclusions are drawn on the basis of ecological data.
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ANTC67: Midterm Notes
¾ The bias occurs because the association observed between variables at the group level does not necessarily represent the association
that exists at the individual level.
¾ Cross-sectional studies measure the prevalence of disease and thus are often called prevalence studies.
¾ In a cross-sectional study the measurements of exposure and effect are made at the same time. It is not easy to assess the reasons for
associations shown in cross-sectional studies.
¾ The key question to be asked is whether the exposure precedes of follows the effect. *
¾ If the exposure data are known to represent exposure before any effect occurred, the data from a cross-sectional study can be treated
like data generated from a cohort study.
¾ Cross-sectional studies are relatively easy and inexpensive to conduct and are useful for investigating exposures that are fixed
characteristics of individuals, such as ethnicity or blood group.
¾ In sudden outbreaks of disease, a cross-sectional study to measure several exposures can be the most convenient first step in
investigating the cause.
¾ Data from cross-sectional studies are helpful in assessing the health care needs of populations.
¾ Frequency of disease and risk factors can then be examined in relation to age, sex and ethnicity.
Box 3.1 WHO Global InfoBase: An Online Tool
¾ The WHO Global InfoBase is a data warehouse that collects stores and displays information on chronic diseases and their risk factors.
¾ The following are available online:
o Compare countries using WHO estimates for certain risk factors
o Make country profiles showing the most recent most nationally-representative data
o Use a survey search tool for all country data on particular risk factors
¾ Case-control studies provide a relatively simple way to investigate causes of diseases, especially rare diseases.
¾ They include people with a disease (or other outcome variable) or interest and a suitable control (comparison or reference) group of
people unaffected by the disease or outcome variable.
¾ The study compares the occurrence of the possible cause in cases and in controls. The investigators collect data on disease occurrence at
one point in time and exposures at a previous point in time.
¾ Case-control studies are longitudinal, in contrast to cross-sectional studies.
¾ Case-control studies have been called retrospective studies since the investigator is looking backward from the disease to a possible
¾ In this sense a case-control study may be either retrospective, when all the data deal with the past, or prospective in which data
collection continues with the passage of time.
Selection of Case of Controls
¾ A case-control study begins with the selection of cases: these cases should represent all the cases in a specified population group. Case
are selected on the basis of disease, not exposure.
¾ Controls are people without the disease.
¾ /[ZoovP]vP}(]v}-effective way to identify and enrol control subjects.
¾ The controls should represent people who would have been designated study cases if they had developed the disease.
¾ Case control studies can estimate relative risk of disease, but they can NOT determine the absolute incidence of disease.
¾ An important aspect of case-control studies is the determination of the start and duration of exposure for cases and controls.
¾ In the case-control design, the exposure status of the cases is usually determined after the development of the disease (retrospective
data) and usually by direct questioning of the affected person or a relative or friend.
¾ Exposure is sometimes determined by biochemical measurement (e.g. in blood or cadmium in urine), which may not accurately reflect
the relevant past exposure.
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