Epidemiological Study Designs overview explanation

What is design mean
Different type of study design

1. Overview of Epidemiological
Study Designs
By Addisu W. (MPH, Epi&Bio)

2. Learning Objectives

•By the end of this session, students
should be able to
–Differentiate types of study design
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3. Overview
What is design???
Arrangement of conditions for the collection & analysis
of data
Logical model that guides the investigator in the
various stages of the research process
Overall structure of the study
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4. Overview…
•Generally study design divided into two
–Observation
–Experimental
•Observation study designs
–Descriptive(case series, case report, cross-
sectional)
–Analytical(cross-sectional, case-control, cohort

5. Observational vs. Experimental
•Experimental
–The investigator assigns individuals to two or more groups that
either receive or do not receive the preventive or therapeutic
agent
•Observational
–The investigator lets nature take its course
•comparing women who choose to drink during pregnancy
with those who decide not

6. Descriptive Study Designs

7. 1. Case report and case series

•Case reports describe the experience of a single
patient
•Case series describe the experience of a group
of patients with similar diagnosis
•Both document unusual occurrences
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8. Limitations of case report/series

•The presence of any risk factor may be
coincidental.
•Interpretation of information from case series is
severely limited by the lack of an appropriate
comparison group.

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9. 2. Cross sectional studies (surveys)
•Examines the relationship between diseases (or
other health related characteristics) and other
variables of interest at one particular time.
•Populations are commonly selected without
regard to exposure or disease status.
•Cross sectional studies are carried out for:
 public health planning

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10. Cross sectional studies (surveys)…
•subjects are sampled without respect to
disease/exposure status and are studied at a
particular point in time.
•The term "cross-sectional study" (or "prevalence
study") usually refers to studies at the individual
level.

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11. Cross sectional studies (surveys)…
•conducted to estimate prevalence is called a
prevalence study
•Need to be conducted using representative
sample

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12. Advantages of cross sectional studies:
•One-stop, one-time collection of data
•Less expensive & easier to conduct
•provide much information useful for planning health
services and medical programs
•show relative distribution of conditions, disease,
injury and disability in groups and populations
•studies are based on a sample

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13. Disadvantages
•Unable to measure the incidence

•Difficult to make a causal inference
•Associations identified might be difficult to interpret
•Unable to investigate the temporal relation between
outcomes and risk factors
•Not good for studying rare diseases
•Susceptible to biases such as nonresponse bias and
recall bias
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14. Observational analytic study design

15. Case Control Study
•Also known as Case-referent study
•Definition
–Subjects are selected on the basis of whether they do (cases)
or do not (controls) have a particular disease under study
–Groups are then compared with respect to the proportion
having a history of an exposure or characteristics of interest.
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17. When Is It Desirable to Use the Case–Control
Method?

•Situations in Which a Case–Control Study Is
Desirable
–Exposure data are difficult or expensive to obtain.
–The disease is rare.
–The disease has a long induction and latent period.
–Little is known about the disease.

18. Selection of cases
•The criteria that are used to make such decisions are
usually based on a combination of:
– signs and symptoms,
–physical and pathological examinations, and
–results of diagnostic tests.
•Which and how many criteria are used to define a case
have important implications for determining accurately
who has the disease.

19. Selection of cases…
•For example, if only chest pain to define cases of MI,
–most but not all heart attack cases would be included
(people with “silent” heart attacks would be missed),
–people with other conditions that produce chest pain
(such as indigestion) would be included mistakenly
•It is better to be restrictive because it leads to
fewer classification errors

20. Sources of cases
1) Hospital or medical care facility
–This approach is referred to as hospital-based case control
study
–is more common because it is relatively easy & inexpensive
to conduct
2) General population
–Referred as population-based case control study
–involves locating and obtaining data from all affected
individuals or a random sample from a defined population


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21. Selection of Controls
•Controls are a sample of the population that
produced the cases.
•Another term for the control group is Referent
group because it “refers to” the exposure
distribution in the source population

22. Selection of Controls
•Involves consideration of a number of issues
including :
the characteristics and source of the cases
the need to obtain comparable information from cases
and controls
practical and economic considerations.
•the control subjects should be selected to be
comparable to the cases

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23. Sources of controls
1)Hospital
2)Population
3)Special controls

24. Sources of information for disease status

•review of death certificates, case registries that
maintain ongoing surveillance
•Office records of physicians
•Hospital admission or discharge records
•pathology department log books

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25. Sources of information about the exposure
from study subjects themselves, by either
interview or mail questionnaire
from a surrogate, such as spouses of
participants or mothers of children
From records (e.g. medical records).

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26. Advantage of case-control study
•Allow for the study of rare disease
–E.g. determinants of Kaposi's sarcoma
–If a disease developed in 1 in 1000 people per year, one
would expect 10 cases in 10 years in group of 1000 people.
•Possible to look at multiple risk factors at once.
–The researcher could ask both cases and controls about
exposure to HIV, asbestos, smoking, lead, alcohol

27. Disadvantage of case-control
•Recall bias
•Not appropriate for rare exposure
•Selection bias
–if all cases of Kaposi's sarcoma were found to come
from community outside a battery factory with high
levels of lead in the environment, then controls from
across the country with minimal lead exposure would
not provide an appropriate control group.

28. Cohort study
•Definitions of cohort
–The term cohort comes from the Latin word cohors, meaning
a group of soldiers.
–cohort
1)Any designated group of persons who are followed or
traced over a period of time.
2)A group of individuals with a common characteristic or
experience

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30. Cohort study cont.…
•Cohort study is the standard term used to describe
an epidemiologic investigation that follows groups
with common characteristics
•Other terms used instead of cohort study are:
Follow up study
Incidence study
Longitudinal study
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31. Sources of exposure information
•Preexisting records
•Information supplied by the study subjects
themselves
•Direct physical examination or testing



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32. Sources of outcome data
•Depend on the specific resources available as well as
the particular disease under investigation
–Death certificates
–Records
–Directly from the study participants
–periodic direct medical examinations

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33. Advantage of cohort study
•Clarity of temporal sequence
•Can compute incidence rate
•Use to study of rare exposure
•Allow to examine multiple effects of a single
exposure

34. Disadvantage of cohort study
•Loss to follow up
•Not appropriate for rare diseases
•Very expensive and time consuming
•Not good for disease with long latency period

35. Experimental Study
•Also called interventional study.
•an investigation involving intentional change
in some aspects of the subjects.
•E.g. introduction of a preventive or therapeutic regimen

36. Key Features of Experimental Design

1)Investigator manipulates the
condition under study
2)Always prospective

37. Challenges of Intervention Studies
•Experiment on human beings is an ethically charged
undertaking
•Potential interventions must be safe and acceptable
for human use
•Dose schedule of potential interventions must be
established

38. THANK YOU!!!