Descriptive Epidemiology hudyvugufvuc🌿- Copy.ppt

Epid

1. Descriptive Epidemiology
By Temesgen T.

2. Objective of the course
• Mention types of study design
•Describe the basic concepts of descriptive
studies
•Describe the concept of time, place and
person in descriptive studies
•Describe how to carry out common
descriptive studies
•Describe the limitations and strength of
Descriptive studies
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3. Epidemiologic study designs
Definition:
•STUDYDESIGN is an arrangement of
conditions for the collection and analysis of
data that leads to the most accurate answer
to the research question and the most
economical way.
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4. •The basic design strategies used in
epidemiologic research can be broadly
categorized according to whether such
investigations focus on describing the
distributions of disease or elucidating its
determinants.
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5. Introduction
Epidemiology: is primarily concerned with the
distribution and determinants of disease in human
population.
Broad categories of epidemiological studies: according to
their focus of investigation
•Descriptive studies: focus on the distribution of disease
•Analytic studies: focus in elucidating the determinants
of disease
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6. Introduction (…cont’d)
Purpose of Epidemiological Studies
Descriptive Analytic
Characterize disease
occurrence by time, place

and person.
Generate testable hypothe
sis as to the cause of diseas
e
Concerned with the
search for causes and effects.
Test hypothesis about
association between
exposure and outcome.
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7. Introduction (cont.)
Types of Epidemiological Design Strategies
Descriptive Analytic
Dealing with population
• Correlational or ecological
Dealing with individuals
• Case report or series
• Cross sectional survey
Observational studies
• Case-control
• Cohort
Intervention studies
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8. Descriptive epidemiology
Descriptive epidemiology is a way of organizing data
related to health and health related events by person
(Who), place (Where) and time (When) in a population.
•Provides information about:
1) The magnitude of the problem,
2) The populations at greatest risk of acquiring a particula
r disease (severity),
and
3) The possible cause(s) of the disease.
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9. Descriptive epid…(cont.)
Time: Information organized by time
Easily shows the trend of the disease over time
and establishes the usual occurrence of the
disease in the population which is essential in
identifying excess occurrence (epidemics).
It can also be used to predict seasonal and
secular (long-term) trends.
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10. Descriptive epid…(cont.)
Place: This provides information on geographic
distribution of the disease.
Provides clue in identifying factors influencing
the
occurrence of the disease either in the host or
environment.
Person: Describing disease occurrence by personal c
haracteristics
Important to identify some modifiable factors in ord
er to prevent or control the disease.
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11. Descriptive epid…(cont.)
Person data include:
The inherent characteristics of people (age,
ethnic group, gender),
Their acquired characteristics (educational,
marital, immune, or nutritional status),
Their activities (occupation, leisure activities,
use of alcohol, tobacco, or medications), or
The conditions in which they live
(socioeconomic status, access to health care).
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12. Descriptive epid…(cont.)
Types of descriptive studies:
• Correlational /ecological
• Case report or case series
• Cross-sectional
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13. Correlational/ecological studies
• Uses data from entire population to compare
disease frequencies
- between different groups during the same
period of time, or
- in the same population at different points in
time.
• Does not provide individual data, rather
presents average exposure level in the
community.
• Cause could not be determined.
• Correlation coefficient (r) is the measure of
association in Correlational studies.
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14. Case Reports (Case series)
Case reports (case series): report or describe the experience of
a single individual or a group of individuals with the same
diagnosis or health problem
 Derived from either the practice of one or more health care
professionals or a defined health care setting ( hospital, health
centre or specialized clinic)

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15. Case Reports (Case series) (cont.)
•Information/data in making better use of the case series
study:
1. Defining the disease or health problem clearly
2. Recording the date when the disease/death occurred (Time)
3. Recording where the person lived, worked,… (Place -
relevant to the study)
4. Recording personal characteristics ( age and sex)
5. Explore the opportunities for collecting additional data
from records or the person directly.
6. Estimating the size and the characteristics of the population
at risk
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16. Case Reports (Case series) (cont.)
Strength:
•Useful for studying signs and symptoms (new
syndromes) and creating case definitions for
epidemiological studies.
Example: AIDS
• Can help build up a picture of the natural history of a
disease.
• Very useful in providing critical information, for
hypothesis generation.
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17. Case Reports (Case series) (cont.)
Limitation:
•Report is based on single or few patients, which could
happen just by coincidence.
•Lack of an appropriate comparison group.
•Rates can not be calculated since the population
corresponding to the source of cases can not be defined
well.
•Detailed and complete risk factor information is difficult
to obtain for all cases from records.
•Studies are prone to atomistic fallacy (the opposite of
ecological fallacy). Example: at an individual level a high
income may be associated with lower rate of suicide but this does
not mean that societies which are rich have a lower rate of suicide.10/27/25 17

18. Cross-Sectional Studies
Cross sectional studies or prevalence survey:
•Measure disease and exposure status simultaneously
among individuals in a well-defined population.
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19. Cross-Sectional Studies (cont.)
application
simultaneously
Data C
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20. Cross-Sectional Studies (cont.)
Characteristics of cross-sectional studies:
•Assess both exposure and outcome simultaneously
•Are based on point prevalence rates with only few
exceptions
•Are frequently made on total population samples
•Subdivision of the total population takes place after data
collection unlike in cohort & case- control studies
•Are cheaper, easier & useful for studies of exposures
that are unalterable over time or relatively permanent
features for individuals E.g.-blood group, ethnicity, sex,
etc.
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21. Cross-Sectional Studies (cont.)
Advantages
•Less expensive: a one-step, one-time collection of data
•It starts with reference population, thus making possible
generalization
•Provide much information useful for planning health
services & medical programs
•Show relative distribution of conditions, Ds, injury &
disability in groups & populations
•Are based in a sample of a major population & do not relay
on individuals that present themselves for medical
treatment.
Disadvantages
•It is difficult to know which occurred first, the exposure or
the outcome, this is known as 'chicken or egg dilemma'.
•It may not show strong cause-effect relationship if sample
size is small.
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22. THANK YOU!
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