2. Lesson objective
At the end of this lesson learners will
be able to :
o classify study designs
o Describe the characteristics of
descriptive and analytic study designs
o Design and describe the strength and
limitations of case report/series,
ecological and cross-sectional study
designs
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3. Study Design
Is a specific plan or protocol for
conducting the study
Direct how the investigation is conducted
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5. Descriptive
Describing the frequency or characteristics of
events
In descriptive epidemiology, we organize and
summarize data according to time, place, and
person.
Not intended to confirm causes/ determinants
May be carried out at the level of individual
and Populations
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6. Correlational or Ecological
study
Uses data from entire population to compare
disease frequencies b/n different groups
during the same period of time, or in the
same population at different points in time
Unit of observation is the group, not the
individual
Does not provide individual data, rather
presents average exposure level in the
community
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7. Measuring correlation studies
Correlation coefficient(r)
• Descriptive measure of association in
correlation studies.
• The extent to which there is linear
relationship between exposure and
disease.
• Value of correlation coefficient is between
+1 and –1
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8. Ecological study …
Example:
Hypertension rates and average per capita
salt consumption compared b/n two
communities
Comparing incidence of dental cares in
relation to fluoride content of the water
among towns
It is important to note that positive
association does not necessarily imply a
valid statistical association
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9. Ecological cont…
Strength
can be done quickly and inexpensively/cheap,
often using available data
Limitation:
Subject to ecological fallacy- association at
group level is not the same as association at
individual level
Inability to link exposure with disease.
Lack of ability to control for effects of
potential confounding factors
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10. Case Report
Careful and detailed report of the profile of a
single patient
Document unusual medical occurrences/
generally report a new or unique finding
(previous undescribed disease, unexpected
new therapeutic effect… )
Can provide clues in identification of a new
disease or adverse effects of exposure
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11. Case Series
Experience of a group of patients with a
similar diagnosis
Description of clinical/epidemiologic
characteristics of a number of patients with a
given disease
Generally report on new/unique condition
Collection of individual case reports occurring
within a fairly short period of time
May be only realistic design for rare disorders
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12. Case series example:
The 5 young homosexual men with PCP seen between Oct.
1980 and May 1981 in Los Angeles created a serious concern
among physicians since PCP among young adults is not
common. Later, with further follow-up and thorough
investigation of the strange occurrence of the cluster of cases
the diagnosis of AIDS was established for the first time.
Advantages
Useful for hypothesis generation
Informative for very rare disease with few established risk
factors
Disadvantages
Cannot study cause and effect relationships
Cannot assess disease frequency
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13. Cross-Sectional Study
An observational design that surveys
exposures and disease status at a single point
in time
Sample without knowledge of Exposure or
Disease
It Measures prevalence, not incidence of
disease- mostly prevalence studies/surveys
Not suitable for studying rare or highly fatal
diseases or a disease with short duration of
expression
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14. Cross-sectional Design
time
Study only exists at this point in time
Study
population
No Disease
Disease
factor present
factor absent
factor present
factor absent
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15. Purpose of Cross-sectional
study
Descriptive
a. How common is the factor of interest (disease,
risk factor, etc.) in the study population?
b. What are the characteristics of the group of
interest in the population?
c. What are the distributions of factors of interest
in the study population?
Associative
a. What are the relationships of the factors of
interest to other factors in the study population?
b. How do persons with the factor of interest differ
from those without it?15
16. Advantages of cross-sectional
study
Good design for hypothesis generation
Can estimate overall and specific disease prevalence
Can estimate exposure proportions in the population
Can study multiple exposures or multiple outcomes
or diseases
Relatively easy, quick and inexpensive
No issue of subjecting any animals or procedure to
particular treatments
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17. Disadvantages of cross-
sectional study
Impractical for rare diseases
Temporal sequence b/n exposure and
disease cannot be established- chicken-
egg dilemma- which came first, chicken
or the egg?
Confounding is difficult to control
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