This document discusses different types of observational study designs used in epidemiology. It describes cohort studies, both prospective and retrospective, as well as case-control studies. Prospective cohort studies follow groups of subjects over time to observe outcomes based on exposures. Retrospective cohort studies use historical data to compare groups. Case-control studies compare cases with a disease to controls without the disease to identify risk factors. Cohort studies can precisely measure exposure-outcome relationships but are time-consuming and expensive. Observational studies are prone to biases like confounding, selection, information, and measurement bias.

1. Observational Study Design

2. Design of studies
• The testing of hypotheses must be done by
making controlled observation, free of systematic
bias.
• Statistical techniques to be valid , must be
applied to data obtained from well designed
studies otherwise solid knowledge is not a
advanced.
Study type
• Observational
• Experimental

3. • Observational-
• Nature determines who is exposed to the
factor of interest and who is not exposed.
• Experimental –
• The investigator determines who is exposed
these may causation.

5. Cross sectional studies
• Cross sectional study measurement are taken
at one point in time
• Example
• Cross sectional study of high blood pressure
and heart diseases at same time.
• It is useful in showing association in providing
easy clue to etiology.

7. Case control studies
• Investigator starts with lung cancer cases and with
control through examination of the record or through
interviews determines the presence or absence of
factors in which he or she is interested [ smoking].
• It also referred as retrospective study.
• The data on the factor of interest are collected
retrospectively and thus may be subject to various
inaccuracies.
• It is useful in rare disease or condition or when the
disease takes a very long time to become manifest.

8. Prospective / cohort study
• The investigator starts with a cohort of non-
diseased person with that factor [ i.e those
who smoke ] and person without that factor [
non smoker ] and goes forward into some
future time to determine the frequency of
development of the disease in the two group.
• It is also known as longitudinal study.
• It is very useful for stronger evidence of
casuality and less subject to bias.

9. • Prospective studies provide stronger evidence of
casualty than retrospective studies but more
difficult , more costly and sometimes impossible
to conduct.
• The disease under study takes decades to
develop or if it is very rare.
• In health field, an experimental study to test an
intervention of some sort is called a clinical trial.
• Clinical trial prospective, experimental studies
that provide the most vigorous evidence of
casualty

10. • Observational studies are an alternative to
experimental studies.
• An observational study is sometimes termed a natural
experiment.
• Instead of being randomized into one group or another
to ensure statistical balance, subjects are classified into
groups either by the presence of an exposure, which is
called a cohort study, or the presence or absence of a
disease, which is called a casecontrol study.
• A subject could be a cell, a bacteria, a specific cell line,
a pond of environmental interest, a rat, or a person.

11. Cohort studies (retrospective):example
• Having been exposed to asbestos in the workplace or
not (or at different levels of asbestos exposure) with
lung cancer as the outcome;
• Growing up in an area with high fluoride water
compared with growing up in an area without much
fluoride in the water with dental caries as an outcome;
• comparing the outcomes of two different treatments
for acne based on a registry of clinic patients in the
past 5 years; or
• Evaluating the effect of childhood obesity on diabetes
using records from 10 years of a pediatric practice.

12. Cohort studies (prospective):
• Choosing to smoke or not to smoke with the
outcome being the development of lung cancer,
• Emphysema, or heart disease; being part of an
ecosystem that is high in volatile organic
compounds (VOCs) compared with an ecosystem
that is low in VOCs with the outcome being
survival of a flora or fauna species;
• Determining whether the apple- or pear-shaped
body type (phenotype) leads to an increase in the
development of heart disease, hypertension, or
diabetes.

13. Case-control studies:
• It is being part of a group that develops a
disease such as lung cancer compared with
members of the group that do not develop the
disease;
• comparing patients who have a new highly
virulent infectious disease of unknown
etiology to subjects without the disease but
living in the same neighborhood to identify
factors associated with the etiology or cause
of the disease; or

14. Case-control studies:
• comparing HIV polymerase chain inhibitor–
resistant HIV to nonresistant HIV in order to
identify characteristics differentiating the two
groups;
• comparing the results of a microarray analysis
applied to cells from cancer patients and non
cancer patients or a microarray analysis
applied to normal cells and cancer cells from
the same subjects.

15. Case-control studies:
• Case-control (genetic association) studies: using cases
that have high blood levels of methotrexate compared
with controls that have low blood levels of
methotrexate to identify which alleles of CYP2E1, an
enzyme that affects the rate of metabolism of various
compounds, relate to this phenotype;
• comparing severe chronic asthma (cases) to normal
children of the same age, gender, and ethnicity to
identify genes (or markers) that are associated with the
disease; taking blood samples from cases and controls
and either using a candidate gene approach or doing a
genomewide scan (1

16. Cohort Studies
• A cohort study is one where two or more groups
of subjects are followed over time to see if they
develop some disease or if some event occurs.
• In an exposure study (occupational or
environmental), the effect of exposure on
multiple outcomes—death, cancer, heart
disease—can be observed.
• There are two types of cohort studies:
prospective and retrospective.

17. Prospective Cohort Studies
Prospective cohort studies (also known as
follow-up studies) follow groups of cells,
animals, or patients with different exposures
until some point in time where something
happens or the study is terminated.
Usually the outcomes of interest (e.g., death)
are specified at the start of the study.

18. Prospective-Cohort studies
In the British Physician study, a prospective study of smoking,
34,439 male British doctors were invited to participate in a study
on the effects of smoking. Initially, there were two groups,
smokers and nonsmokers. Eventually, a third group, those who
quit smoking, was followed for 10 years, then 20 years, and
recently the 50-year follow-up was reported (7). They were
followed to observe what diseases would develop related to
smoking status. The risk of lung cancer for smokers was
2.49/1000, whereas the risk of lung cancer for nonsmokers was
0.17/1000. Thus the relative risk of lung cancer for smokers over
a 50-year period is 14.7

19. Retrospective Cohort Studies
Retrospective cohort studies use
historical data to make comparisons
based on risk factors or exposures that
occurred prior to the event. Historical
records of snowfall in different
continents can be used to study the
effects of global warming.

20. Advantage of cohort study
• It precisely indicate the chronological
sequences between exposure and outcome.
• It helps to calculate incidence of disease in
exposure group
• It allow examination of a multiple effect of a
single exposure.
• It reduces the selection bias

21. Disadvantage of prospective cohort
studies
• Investigators have to follow large number of
subjects for a long time.
• Very expensive and time consuming
• Not good for rare diseases.
• Differential loss to follow up can introduce
bias

22. Disadvantage of retrospective cohort
studies
• They are not good for rare disease
• Records not part of design is used than
available of data is poor.
• Absence of critical factor if data was recorded
in the past.
• Differential loss to follow up can introduce
bias.

23. Limitation of cohort studies
• It is unsuitable for identifying the causes of a
sudden outbreak of disease.
• Expensive to run
• It take many years to produce results.
• It offers only clues not the definite proof
about the causes of diseases.
• Participants may leave the cohort, move away
out of contract or die. All these can bias the
results.

24. Bias in observational studies
• Confounding bias
• It is an additional risk factor that affects the
outcome
• Let us consider alcohol and smoking
• It is belief that people who drink alcohol lead to
cancer .
• It is known factor that smoking cause cancer
• Some people drink alcohol at same time smoking
too. Thus smoking is the confounder between the
alcohol and cancer.

25. Selection bias
• It is problem in observational studies.
• Groups are not randomised
• Groups are assigned as per investigator wish.

26. Information bias
• The inaccurate information collected from
subjects, results in giving the wrong outcome.
• This inaccurate information may come from
the either subject or personnel

27. Protopathic bias
• Selection of diseased participant is bias
• Disease may appear after the drug intake
causes to misleading of the study.

28. Measurement bias
• If many groups are involved in the study, the
personnel making measurement and the
faulty instruments used are the two major
problems that cause the bias