The document discusses epidemiological study designs, focusing on descriptive and analytical studies, their purposes, strengths, and limitations. It describes how descriptive studies characterize disease occurrence by person, time, and place and serve to generate hypotheses for further analytical studies. Various types of descriptive studies including case reports, case series, cross-sectional studies, and correlational studies are outlined with their specific methodologies and applications in research.

1. Epidemiological Study Designs
By Tilaye Workneh
(MPH/Epid, Asst. Prof. of Epidemiology)
Lecture 4

2. Objective of the course
3/24/2024
 Introduction to study design
 Describe the basic concepts of descriptive studies
 Describe when and how to carry out common descriptive studies
 Describe the limitations and strength of Descriptive studies

3. 3/24/2024
 There are ample of health problem in the community which need
evidence based answer
 Planners and decision makers also need evidence as a resource based
on magnitude and factors causing disease for prevention and control
 Health professionals and researchers need evidence to answer why
and how the disease occurs
 Thus, all need research . . . appropriate study design
Introduction

4. Introduction
 Research design is the overall plan which connect the conceptual
research problems to the pertinent research outcome
 Study design is action plan which portray the strategies and tool
to answer the research question
 Study Design articulates what data is required, what methods are
going to be used to collect & analyze data, & how all of these are
going to answer your research question
3/24/2024

5. Introduction …
3/24/2024
 Epidemiology is broadly categorized into descriptive and analytical
=> Corresponding action plan /design/ exist
 Broad categories of epidemiological studies: according to their
focus of investigation
1. Descriptive studies: focus on the distribution of disease /person,
time and place/
2. Analytic studies: focus in elucidating (ማብራራት) the
determinants of disease /analyze cause effect relationships/

6. Introduction . . .
Purpose of Epidemiological Studies
Descriptive study Analytic Study
 Characterize disease occurrence
by person, time and place
 Generate hypothesis
 Dealing with population
－ Correlational or ecological
 Dealing with individuals
－ Case report
－ Case series
－ Cross sectional
survey/Prevalence study
 Concerned with the search for
causes and effects relationships
 Test hypothesis about association
between exposure and outcome
 Observational studies
－ Case-control
－ Cohort
 Intervention studies
－ RCT
3/24/2024

7. Introduction …
3/24/2024

8. Descriptive Epidemiology
3/24/2024
 Description of disease occurrence by person, place and time
 Not intended to asses causality
 The Purpose of descriptive epidemiology is to quantify amount
and characterize the distribution of disease within a population.
Thus, in other words to identify health problems and patterns of
disease occurrence

9. Summary on Purpose of Descriptive Epidemiology
3/24/2024
 Trend analysis
 Basic to health policy
－ Measuring health status
－ Assessing needs
－ Identification of priorities
－ Health planning and evaluation
 Cue about cause

10. Descriptive study designs …
3/24/2024
 Describe the person, time and place of health event occurrence
 Descriptive studies generally precede analytic studies,
 Descriptive information is important to generate hypothesis that
should be tested by analytical study designs
 Thus, descriptive studies often help to generate hypotheses about
cause and effect relationship

11. Descriptive study designs …
3/24/2024
Person
 Since disease does not occur at random:
 What kinds of people tend to develop a particular disease, and who tends
to be spared?
 What’s unusual about those people?
－Age,
－Sex,
－Race/Ethnicity,
－Social class,

12. Person…
3/24/2024
Age
 the most fundamental factor to consider when describing disease
occurrence
 The incidence of most chronic diseases increases with age
 However, the incidence of many infectious diseases is highest in
childhood

13. Person…
3/24/2024
 Sex:-biological and non-biological factors related to sex have disease risk.
 In all developed countries, life expectancy is higher in females than males
– principally due to lower heart disease mortality
 However, many chronic diseases occur more frequently in women
(depression, lupus, etc.)
 As lifestyles continue to become more similar, a question is whether
mortality rates will become more similar? (i.e. environmental vs biology)

14. Person…
3/24/2024
Race/Ethnicity – difficult to define, and to identify which characteristics
may relate to disease occurrence
- Remarkable variation exists in rates of disease occurrence across racial and
ethnic groups
－ Genetics?
－ Socioeconomic status(SES)?
－ Environmental exposures?
－ Access to health care?
－ Lifestyle factors, nutrition, …. ?

15. Person…
3/24/2024
Social class –Despite its unreliability, SES is consistently
associated with mortality in a gradient fashion
Example:
 Occupation
 Education
 Area of residence
 Income
 Lifestyle

16. Place
3/24/2024
Investigation by place includes:
 Across countries (international)
 Within country variation
 Urban/rural differences
 Localized areas

17. Place…
3/24/2024
 Infectious and chronic diseases show great variation from one
country to another
 Some differences may be attributed to:
－ Climate
－ Cultural factors
－ Diet
－ Genetics

18. Place…
3/24/2024
Some differences in disease occurrence between urban and rural
locations may be attributed to:
 Diet
 Physical activity
 Housing conditions (i.e. lead paint)
 Crowding (i.e. spread of infection)
 Pollution

19. Time
3/24/2024
 Since disease does not occur at random:
 How does disease frequency change over time, and what other
factors are temporally associated with those changes?
 The occurrence of health-relate events can vary by time:
 Secular trends
 Cyclic fluctuations (periodic)
 Point epidemics

20. Time…
3/24/2024
Secular trends
 refer to gradual changes in disease occurrence over long periods of
calendar time.
Example: Death from communicable diseases has been declining
where as deaths from non-communicable diseases has been
raising in some developing countries
Cyclic fluctuations /periodic/
 refer to shorter-term increases and decreases in disease occurrence
over a period of years, or within a year

21. Descriptive Study Designs
3/24/2024
 Studies based on individual level information
 Case reports
 Case series
 Cross sectional surveys
 Studies based on population level information
 Ecologic/correlational (aggregate) studies

22. 1. Case Reports
3/24/2024
 Careful and detailed report by one or more clinicians about the profile
of a single patient
Purpose of case report
－ Document unusual medical occurrences
－ Can provide clues in identification of a new disease or adverse effects of
exposures (i.e. case report gave the clue OC use increases the risk of venous
thromboembolism)
 Case report is compiled from patient history, physical examination
and Laboratory investigation, radiology investigation, Treatment,
Outcome, etc.

23. Case report…
3/24/2024
 A group of physicians present a careful and detailed profile of a
single case which has not been seen before or change in natural
history of disease
 This may provide a clue as there is
 An emergence of new disease
 A change in the natural history of a disease
 A change in the agent character
 Emergence of new strain

24. Case report…
3/24/2024
Limitations of Case report
 No appropriate comparison group
 Cannot be used to test for presence of a valid statistical association
 Since based on the experience of one person:
－presence of any risk factor may be purely coincidental
－not a true epidemiologic design

25. 2. Case Series
3/24/2024
 Description of clinical/epidemiologic characteristics of a number
of patients (usually 5-12) with a given disease
 Collection of individual case reports occurring within a fairly
short period of time
Purpose of case rseries
 Similar to case report
 Document unusual medical occurrences
 Can provide clues in identification of a new disease or adverse
effects of exposures
 Upcoming of epidemic

26. Case series…
3/24/2024
Strengths of case series:
 Used as an early means to identify the beginning or presence of an
epidemic
 Can suggest the emergence of a new disease (i.e. AIDS)

27. Case series…
3/24/2024
Limitations of case series:
 Lack of an appropriate comparison group
 Cannot be used to test for presence of a valid statistical association
 Not a true epidemiologic design
 Studies are prone to atomistic fallacy (opposite of ecological
fallacy) –impossible to generalize the result to population
Example: At individual level a high income may be associated with
lower rate of suicide but this does not mean that rich societies have a
lower rate of suicide
representativeness of cases???

28. 3. Cross-Sectional Studies
3/24/2024
Purpose of cross-sectional study
 Snapshot of the health status of population at a certain
point/cross section/ in time
 For each subject, exposure and disease outcome are assessed
simultaneously at cross section of time (hence, called a
“prevalence study/survey”)
 Can also used to compare prevalence of outcomes among
exposed and non-exposed => Analytic characteristics

29. Cross-Sectional study …
3/24/2024
Strengths:
 Provides prevalence estimates of exposure and disease for a well-defined
population such as burden of disease, risk factors, coverage of
interventions, health service utilization, knowledge, attitude and
practice
 Easier to perform than studies that require follow-up (hence relatively
inexpensive)
 Can evaluate multiple risk (and protective) factors and health outcomes at
the same point in time

30. Cross-Sectional study …
3/24/2024
Strengths:
 May identify groups of persons at high or low risk of disease
 Can be used to generate hypotheses about associations between
predictive factors or exposures and disease outcomes

31. Cross-Sectional study …
3/24/2024
 Limitations of cross-sectional study:
 Prevalent rather than incident (new) cases are used
 Can not show strong cause and effect relationship
－ Statistical significance
－ Temporal sequence b/n exposure and disease cannot be
established
* i.e. Which came first, chicken or the egg?
 Cross-sectional study will over represent cases with long duration &
under represent with short duration of illness
 People who died of the disease are missed in cross-sectional study

32. Cross-Sectional Studies…
3/24/2024
Example:
Respiratory problems
Yes No Total
Smoking Yes 70 50 120
No 30 70 100
Total 100 120 220
 Prevalence of smokers among respiratory problems
/prevalence of exposure/
=70 x100=70%
100
 Prevalence of respiratory problems among smokers
/prevalence of outcome/
=70 x100=58.3%
120

33. Analysis in Cross sectional study
 Descriptive statistics , chi-square, regression (linear, logistic)
 Measure of association like Odds ratio/prevalence ratio…the
prevalence of the disease is higher among those exposed
3/24/2024

34. 4. Correlational/ecological studies
3/24/2024
 Purpose of correlational or 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
and outcome in the community level
 Cause could not be ascertained or measured
 Correlation coefficient (r) is the measure of association in
correlational or ecological studies

35. 4. Correlational/ecological studies…
3/24/2024
Example:
－ Average per capita fat consumption and breast cancer rates
compared between countries.
－ Comparing incidence of dental carries in relation to fluoride content
of the water among towns in the rift valley.
－ Mortality from CHD in relation to per capita cigarette sales among
the regions of Ethiopia.

36. Breast Cancer Mortality and Dietary Fat
Intake
3/24/2024

37. Forecast of Cancer Deaths
3/24/2024
Forecast of cancer deaths if present trends continue
(Data from the American Cancer Society)

38. Possible Reasons for Changes in Trends
3/24/2024
Artefactual
 Errors in numerator due to
 Changes in the recognition of disease
 Changes in the rules & procedures for classification of causes
of death
 Changes in the classification code of causes of death
 Changes in accuracy of reporting age at death
 Errors in the denominator due to
 Error in the enumeration of the population

39. Possible Reasons for Changes in Trends …
3/24/2024
Real
 Changes in age distribution of the population
 Changes in survivorship
 Changes in incidence of disease resulting from
 Genetic factors
 Environmental factors

40. Correlational/ecological studies …
3/24/2024
Strength of correlational/ecological study
 can be done quickly,
 inexpensively, and
 often using available data (routine records and reports– e.g.
death rates, per capita income, national food consumption…)

41. Correlational/ecological studies …
3/24/2024
Limitation of correlational/ecological study
 Inability to link exposure with disease
－ Data on exposure and outcome are not linked at the individual level; association
found with aggregate data (average values) may not apply to individuals (Prone to
ecological fallacy)
 Lack ability to control effects of potential confounding factors.
 It may mask a non-linear relationship between exposure and
disease.
E.g. non-linear relationship in alcohol consumption and mortality from
Congestive Heart Disease

42. Assignment
 1. Search research article on each design and explain
how appropriate for the research question, how is the
design outlines the strategies and tools to answer the
question?
 2. Write tree research titles which need each study
design
 Present to class next session !!! (5 minute for each student)
 Do not copy from each other
3/24/2024