introduction to epidemiological study designs

1. Epidemiological study designs
Abdirisak Mohamed Abdi

2. objectives
• Identify basic study designs used in epidemiology
studies.
• Understand the concept of descriptive
epidemiological studies.
• Understand the concepts of exposure, outcome and
risk.
• Identify the association between exposure and
outcome.

5. introduction
Study design is the arrangement of
conditions for the collection and analysis of
data to provide the most accurate answer to
a question in the most economical way.
Epidemiological study design directs how
investigation is conducted .
Epidemiological studies can be classified
into descriptive and analytical study designs
.

6. Descriptive and analytical study
designs
 Descriptive studies involve the systematic collection and
presentation of data to give a clear picture of a particular
situation and can be carried out on a small or large scale.
 Descriptive study is used to describes Disease occurrence
in a population e.g.
 Incidence
 Prevalence
 Survival
 ANALYTICAL STUDIES
 An Analytical Study attempts to establish causes or risk factors
for certain problems. This is done by comparing two or more
groups, some of which have or develop the problem and some
of which have not.
 Analytical studies is used to determine etiology of disease

7. Randomized
Controlled
Trials
Field trials
Community
Trials5.Longitudinal

9. RISK FACTORS
Social
Cultural
BehavioralBiological
Physical

10. 10
Association between exposure and
outcome
Exposure Outcome
Cause Effect

11. Descriptive Study
Descriptive study is concerned with the following questions:
a. When is the disease occurring (Time)
b. Where it is occurring (Place)
c. Who is getting the disease (Person)
Descriptive study include:
• Case studies
• Case series
• Ecological /correlation
• Cross sectional
• Longitudinal

12. Uses of descriptive study
Provides data regarding
• The magnitude of the disease load
• The types of disease problem in the community in term of morbidity
and mortality rate and ratio
Provides “clues” to the disease etiology
Helps in
• Formulation of etiological hypothesis
 Helps in planning, implementation and evaluation of health
services/programmes

13. Procedures of descriptive study
1.Defining the population to be studied
• Descriptive study are investigation of population not
individual; include total number
• Sex
• Age
• Occupation
• cultural
2. Defining the disease under study
• Specified the disease to be investigated
• The main objective to obtain accurate estimate of disease
in a population

14. 3.Describing the disease .
• The main primary objective of descriptive epidemiology is
to describe the occurrence and distribution of disease by
• Time
• Place
• Person
4. Measurement of the disease
• Cross Sectional Study
• Longitudinal Study
• The information should be available in terms of
• Mortality
• Morbidity (incidence and prevalence)
• distribution

15. 5.Comparing with known indices
Making comparisons between different population, this is to
identify groups who are at risk for certain diseases
6. Formulation of an etiological hypothesis
Hypothesis should formulated in a manner that that it can
be tested
• Hypothesis should specify the following
• -the population- the characteristics of the persons to
whom the hypothesis to be applied
• -the specific cause being considered
• -the expected outcome
• -the dose-response relationship
• -the time-response relationship (time between exposure
to the cause observed)

16. Analyses of descriptive study
• •Data checked for any errors and outliers prior to
analysis.
• •Data explored graphically e.g. plot the frequency
distributions of various variables
• –Check normality of the distribution
• •Standard descriptive statistics
• –Mean, median, quartiles, mode
• –Range, interquartile range, standard deviation
• –Standard error and confidence intervals
• –Prevalence rates

17. (cont.) Analysis of descriptive study
studies
•Association can be explored using correlation and
regression for continuous variables
• –Shows variables are associated, not necessarily
imply a cause and effect relationship.
•Means can be compared
•Complex multivariate analysis (multiple and
logistics regression) can be carried out
• to investigate how a dependent variable is related
to more than one explanatory variable.

18. Cross sectional studies
• Based on a single examination of a cross section
of population at one point in time, results of
which can be projected on the whole population
provided the sampling has been done correctly.
• In this study design information about the status
of an individual with respect to presence/absence
of exposure and diseased is assessed at a point in
time.
• Cross-sectional studies are useful to generate a
hypothesis rather that to test it
• For factors that remain unaltered overtime
(e.g. sex, race, blood group) it can produce a valid
association

19. Uses of cross-sectional study
a. More useful in chronic diseases
b. To find more about disease rather than its etiology
• Prevalence
• Disease outbreaks
• Trends in diseases (repeated c/s studies)
• Risk factors for diseases (e.g., NCDs)
c. Assessing healthcare needs of populations.
d. Describe communities
e. Assess population needs
f. Evaluate programs
g. Establish baseline data prior to the initiation of
longitudinal studies

20. Advantages of Cross-sectional study
• Cheap, rapid, easy
• Can use large sample of the population.
• May study several outcomes
• Control over selection of subjects
• Control over measurements
• Relatively short duration
• Yields prevalence
• Assess health status, and health problems and indicate
priorities for health care planning. Assess customer’s
satisfaction for health care.
• Provides the base-line data for further studies if the
problem is not studied before.
• Cross sectional study is the most convenient first step in the
investigation of the cause of the outbreak or epidemic

21. Disadvantages of Cross-sectional study
• Does not establish cause/effect ratio
• Potential bias in measuring exposure
• Potential survival bias
• Not feasible for rare disease
• Does not yield incidence

22. Longitudinal Study
• Based on multiple observations in the same
population over a prolong period of time.

23. Uses of Longitudinal Study
• Natural History of Disease
• Identifying Risk factors
• Finding out incidence rate

24. Case study
•Detailed presentation of a single case or handful
of cases
•Generally report a new or unique finding e.g.:
• –previous undescribed disease
• –unexpected link between diseases
• –unexpected new therapeutic effect
• –adverse events
•The case may be an individual, an event, a policy,
etc

25. Case study example
• A man suffered from an Myocardial infarction
at an young age and survived. What are his
questions?
– Why did it happen to me?
– Can I prevent a second attack?
– Are my children at higher risk?
– What can be done to lower my risk?
– What can be done to lower my children’s risk?

26. Case report
Case report
• unit of study: single person with a
disease
• limitation: based on experience of a
single person
• provides first clues in the identification
of a disease or adverse effects of
exposure e.g (halothane induced
hepatitis

27. Case series
Case series :
Unit of study: group of persons with a similar diseases
•Experience of a group of patients with a similar diagnosis
•Assesses prevalent disease
•Cases may be identified from a single or multiple sources
•Generally report on new/unique condition
•A realistic design for rare disorders
• Retrospective look at series of cases that have features in
common
•Common diagnosis, treatment, measures
•Each case may be separately described, or the cases may be
lumped together with data summaries

28. Uses of case series
 Formulation of criteria for diagnosis
 Formulation of indications for treatment
 Identification of prognostic factors
 Determination of survival rates

29. Advantages and disadvantages of case
series/case study/ case reports
• Advantages
•Useful for hypothesis generation
•Informative for very rare disease with few established risk factors
•Characterizes averages for disorder
Disadvantages
•Cannot study cause and effect relationships
•Cannot assess disease frequency
• Sequence of events cannot be ascertained
• Not useful for diseases of short period of time
• Not practical in studying rare diseases
• Case reports are susceptible to bias
• Case reports can not be used to make treatment decisions

30. Ecological study designs
• Ecological study also known as correlation study
examine associations between exposure and
disease measures on the population level
• In an ecological study, the units of measurements
are groups of people rather than individuals.
• The incidence of disease in one geographical area
is compared to that of another area. For example
a cancer mortality in areas with hazardous waste
sites as compared to areas without waste sites.