6. Learning Objectives
When you have completed this session you will be
able to:
Describe the differences between and descriptive
and analytic studies
Describe the differences between a case report and
a case series
Describe the characteristics of an ecological study
Describe a cross sectional study and explain its
advantages and disadvantages
Explain the uses of the descriptive study types
7. Descriptive versus Analytical
epidemiology
Descriptive epidemiology:
• generates idea(s) or hypothesis for
associations between risk factor and
illness
Analytical epidemiology:
• uses a comparison group to
establish an association between risk
factors and illness in the two groups
8. Descriptive Studies
The most frequent design strategy found in
the epidemiologic literature
Used to describe the distribution of disease
by time, place, person and assoc. factors
Describing these factors does not link them
However we can identify unusual distributions
or correlations
Useful for Hypothesis generation and health
planning
9. Using Descriptive Studies for
Hypothesis Formulation
Person – “Who is getting the disease?”
Age, race, sex
Place – “Where are the rates of disease
highest and lowest?”
Time – “ When does the disease occur
commonly or rarely?” and “ Is the frequency
of the disease now different from the
corresponding frequency in the past?”
11. Categories of Descriptive Studies
Populations (correlational or ecological studies)
Individuals
Case reports
Case series
Cross-sectional surveys
12. Correlational or Ecological Studies
Based on aggregate measures of exposure and
outcome from several populations.
The population is the unit of observation available
for study.
Exposures:
- What percent of a population smokes?
- What percent of 1-year old children are
vaccinated against measles?
- What percent of a population has piped water?
13. Correlational or Ecological Studies
Outcomes:
- What percentage of a population died from MI?
- What percentage of children had measles last
year?
- What percentage of population had episodes of
diarrhea?
14. Correlational or Ecological Studies
Advantages
-Easy to do
-Use available data (“administrative” or other
aggregate data)
-Can be done in population with widely differing
characteistics
-Generate hypotheses for additional study
15. Correlational or Ecological Studies
Disadvantages
-Unable to examine data for individuals; data on
exposure and data on outcome are collected
independently
-No assurance that persons with exposure (risk
factor) of interest are the same ones with the
outcome (disease) of interest
-Association at the aggregate level may not reflect
association at the individual level - the ecologic
fallacy
-Unable to adjust for potential confounding factors.
-Poor correlation doesn’t mean no association
16. Descriptive Studies
Populations (correlational or ecological studies)
Individuals
Case reports
Case series
Cross-sectional surveys
17. Case reports
The individual is the unit of observation
available for study.
Clinical case with “unusual” clinical picture
May suggest an etiological association
18. Case series
First case report may stimulate compilation
of additional case reports….a case series
(e.g. occurrence of Pneumocystis carinii
among a group of young, homosexual men
with no history of immune deficiency)
19. Case reports or Case series
Advantages:
Use available clinical data
Detailed individual data
Suggest need for investigation (hypothesis
generation)
Disadvantages:
May reflect experience of one person or one
clinician
No explicit comparison group
20. Descriptive Studies
Populations (correlational or ecological studies)
Individuals
Case reports
Case series
Cross-sectional surveys
21. Design of cross-sectional study
Defined population
Exposed: Have
disease
Exposed; Do
not have
disease
Not exposed;
Have disease
Not exposed:
Do not have
disease
Gather data on exposure and disease
23. Cross-sectional study
Also known as “prevalence” study
Design
Identify research question
Specify target and accessible population
Sample the population
Measure variables of interest (usually a survey)
Thus: participants classified by exposure and disease status
at the same time. This allows identification of prevalent
cases, calculation of prevalence rates.
24. Cross-sectional surveys
Measure variable(s) at a single time:
prevalence studies (“snapshot”)
useful for events/diseases when
chronic
common
non-fatal
temporal relationship cannot be established unless
exposure permanent
If exposure unalterable, cross-sectional survey ≈
analytical study
25. Cross-sectional study
Strengths/Advantages
Can study entire populations or a representative
sample
Provide estimates of prevalence of all factors
measured
Standardized data collection tool.
May be quick and inexpensive
Valuable in assessing health status and health
care needs of a population
Can be repeated to get trend data
Help in hypothesis generation
26. Cross-sectional study
Weaknesses/disadvantages
Information on disease and exposure collected
simultaneously, therefore difficulty establishing
that cause antedated effect.
Use of prevalent cases means data reflects
determinants of survival as well as etiology
Cases may be misclassified due to changes in
exposure or poor memory of earlier exposures
Not good for rare diseases or exposures
Cannot measure risk
Cant study temporal relationship
27. Data analysis and interpretation of
descriptive studies
Cross-sectional studies and surveys are
measuring prevalence
Well-suited for describing variables and their
distributions – Eg. Kenya Demographic and
Health Survey
28. Design of a cross-sectional study
Disease No Disease
Job A
Job B
a b
c d
Prevalence of disease in exposed (Job A) = a/a+b
Prevalence of disease in unexposed (Job B) = c/c+d
29. Presentation of Cross Sectional Data
2x2 table
Exposed
Not exposed
ill not ill
a
c
b
d
Prevalence in exposed (Pe+) = a/
(a+b)
Prevalence in non-exposed (Pe-) = c/
(c+d)
Prevalence ratio = a/(a+b) / c/(c+d)
31. Association Measures in Cross
Sectional Studies
Example: Corporal hygiene and trachoma
Poor Hygiene Trachoma Healthy Total
Yes 54 337 391
No 50 1 459 1 509
Total 104 1 796 1 900
Prevalence ratio = 13.8 / 3.3 = 4.2
Prevalence
13.8 %
3.3 %
% exp 51.9% 18.8%
32. Recap
Now that you have completed this session you will be
able to:
Describe the differences between and descriptive
and analytic studies
Describe the differences between a case report and
a case series
Describe the characteristics of an ecological study
Describe a cross sectional study and explain its
advantages and disadvantages
Explain the uses of the descriptive study types
33. What is the prevalence of trachoma, and is it
associated with poor hygiene?
Population of 1900
Poor hygiene Trachoma
Yes No
Yes 54 337
No 50 1459
34. Exercise
Calculate Prevalence of Chlamydia in this
population of STI patients.
Calculate prevalence ratio for Chlamydia
among OCP users vs. non-users.
Editor's Notes
Please do not change the learning objectives without notifying the team.
Move any learning objectives that you don’t expect to cover in class to the “What’s next” slide at the end of the presentation.
Optional slide-delete if irrelevant to your topic
Optional slide-delete if irrelevant to your topic
Optional slide-delete if irrelevant to your topic
Please do not change the learning objectives without notifying the team.
Move any learning objectives that you don’t expect to cover in class to the “What’s next” slide at the end of the presentation.
