A retrospective cohort study examines existing data to investigate associations between exposures and outcomes without prospective follow-up. The document discusses: 1) Retrospective cohort studies identify exposed and unexposed groups from past data and determine current disease status, requiring less time than prospective studies. 2) Limitations include potential for poor quality or incomplete past data, and lack of information on confounding factors. 3) As an example, a study used employee health records to retrospectively examine the association between chemical exposure in tire manufacturing and mortality. However, past data may not fully account for smoking, diet, or other risk factors.

1. RETROSPECTIVE COHORT STUDY
Dr.Bharat Kalidindi
MPH 1st Sem
Padmashree School of Public Health. Bangalore

2. WHAT IS COHORT
A cohort is any group of people who are linked in some way or the
other.
A cohort is a group of subjects who have shared a particular event
together during a particular time span
OR
OR
A cohort is a group of people who share a common characteristic or
experience within a defined period

3. EXAMPLE
• People born in India between 1918 and 1939.
• Survivors of an bus accident.
• Truck drivers who smoked between age 30 and 40.
• People who are exposed to a drug or vaccine or pollutant.

4. WHAT IS A COHORT STUDY
• A cohort study is a form of longitudinal study (a type of observational
study) used in medicine, social science, actuarial science, business
analytics, and ecology.
• In a cohort study, a group of individuals exposed to a putative risk
factor and a group who are unexposed to the risk factor are followed
over time (often years) to determine the occurrence of disease.
• The incidence of disease in the exposed group is compared with the
incidence of disease in the unexposed group.

5. Defined
population
Begin with
Exposed
Not
Exposed
Develop
Disease
Do not
Develop
Disease
Do not
Develop
Disease
Develop
Disease
Identify
Follow

6. • Cohort studies may be prospective or retrospective.
• Prospective cohort study (concurrent cohort study or longitudinal
study): Subjects have been followed up for a period and the
outcomes of interest are recorded.
• Retrospective cohort study (Non-concurrent cohort or historical
cohort study): Study done after the occurrence of both the
exposure and outcome.

7. AN AMBIDIRECTIONAL COHORT STUDY
• A cohort study may also be ambidirectional , meaning that there
are both retrospective and prospective phases of the study.
Ambidirectional studies are much less common than purely
prospective or retrospective studies, but they are conceptually
consistent with and share elements of the advantages and
disadvantages of both types of studies

8. • The Air Force Health Study (AFHS) - also known as the Ranch Hand
Study - was initiated by the U. S. Air Force in 1979 to assess the
possible health effects of military personnel's exposure to Agent
Orange and other chemical defoliants sprayed during the Vietnam
War. The study was conducted comparing:
• 1,098 pilots exposed to dioxin in Vietnam (Operation Ranch Hand)
• 1,549 men who flew cargo missions in Southeast Asia during the
same time

9. ANALYSIS OF COHORT STUDIES
Analysis of a cohort study uses either the risk or the rate ratio
of disease in the exposed cohort compared with the rate or
risk in the unexposed cohort.
Rate Ratio = Incidence rate in exposed group (r1)
Incidence rate in unexposed group (r0)

10. FIRST, IDENTIFY
EXPOSED a+b
NOT
EXPOSED c+d
TOTAL

11. THEN, FOLLOW TO SEE WHETHER
EXPOSED a b a+b
NOT
EXPOSED c d c+d
TOTAL
Disease
does not
develop
Disease
develops

12. CALCULATE AND COMPARE
EXPOSED a b a+b
a/a+b
NOT
EXPOSED c d c+d
c/c+d
Disease
develops
Disease
does not
develop
TOTAL
Incidence
of disease
a/a + b = Incidence in exposed c/c + d = Incidence in not exposed

13. WHEN IS A COHORT STUDY WARRANTED?
• When the (alleged) exposure is known
• When exposure is rare and incidence of disease among
exposed is high (even if the exposure is rare, determined
investigators will identify exposed individuals)
• When the time between exposure and disease is relatively
short
• When adequate funding is available
• When the investigator has a long life expectancy

14. STRENGTHS AND WEAKNESSES OF COHORT STUDIES
Strengths:
• Multiple outcomes can be measured for any one exposure.
• Can look at multiple exposures.
• Exposure is measured before the onset of disease (in prospective
cohort studies).
• Good for measuring rare exposures, for example among different
occupations.
• Demonstrate direction of causality.
• Can measure incidence and prevalence.

15. Weaknesses:
• Costly and time consuming.
• Prone to bias due to loss to follow-up.
• Prone to confounding.
• Participants may move between one exposure category.
• Knowledge of exposure status may bias classification of the
outcome.
• Being in the study may alter participant's behaviour.
• Poor choice for the study of a rare disease.
• Classification of individuals (exposure or outcome status) can be
affected by changes in diagnostic procedures.

16. RETROSPECTIVE STUDY
• In a retrospective study, the outcome of interest has already
occurred at the time the study is initiated.
• A retrospective study design allows the investigator to formulate
ideas about possible associations and investigate potential
relationships, although causal statements usually should not be
made.

17. An investigator conducting a retrospective study typically utilizes
administrative databases, medical records, or interviews with
patients who are already known to have a disease or condition. In
general, the reasons to conduct a retrospective study are to:
• Study a rare outcome for which a prospective study is not feasible.
• Quickly estimate the effect of an exposure on an outcome.
• Obtain preliminary measures of association.
A retrospective cohort study allows the investigator to describe a
population over time or obtain preliminary measures of association
to develop future studies and interventions.

18. Defined
population
Exposed
Not
Exposed
Develop
Disease
Do not
Develop
Disease
Do not
Develop
Disease
Develop
Disease
1989
2015

19. • The investigators jump back in time to identify a cohort of
individuals at a point in time before they have developed the
outcomes of interest, and they try to establish their exposure status
at that point in time. They then determine whether the subject
subsequently developed the outcomes of interest.
• The Distinguishing feature of a retrospective cohort study is that the
investigators conceive the study and begin identifying and enrolling
subjects after outcomes have already occurred.

21. Suppose investigators wanted to test the hypothesis that working with the
chemicals involved in tire manufacturing increases the risk of death. Since this is
a fairly rare exposure, it would be advantageous to use a special exposure cohort
text annotation indicator such as employees of a large tire manufacturing
factory. The employees who actually worked with chemicals used in the
manufacturing process would be the exposed group, while clerical workers and
management might constitute the "unexposed" group. However, rather than
following these subjects for decades, it would be more efficient to use employee
health and employment records over the past two or three decades as a source
of data. In essence, the investigators are jumping back in time to identify the
study cohort at a point in time before the outcome of interest (death) occurred.
They can classify them as "exposed" or "unexposed" based on their employment
records, and they can use a number of sources to determine subsequent
outcome status, such as death

22. • Retrospective cohort studies like this are very efficient for studying rare
or unusual exposures, but there are many potential problems here.
• Sometimes exposure status is not clear when it is necessary to go back in
time and use whatever data is available, especially because the data
being used was not designed to answer a health question.
• Even if it was clear who was exposed to tire manufacturing chemicals
based on employee records, it would also be important to take into
account (or adjust for) other differences that could have influenced
mortality, i.e., confounding factor.
• For example, it might be important to know whether the subjects
smoked, or drank, or what kind of diet they ate.
• However, it is unlikely that a retrospective cohort study would have
accurate information on these many other risk factors.

23. Investigator:
Uses existing data collected in the past to identify the population and the
exposure status (exposed/not exposed groups).
Determines at present the (development) status of disease.
Investigator spends a relatively short time to:
Assemble study population (and the exposed/not exposed groups) from
past data.
Determine disease status at the present time (no future follow-up).

24. DISADVANTAGES OF RETROSPECTIVE COHORT STUDIES
• As with prospective cohort studies, they are not good for very rare
diseases.
• If one uses records that were not designed for the study, the
available data may be of poor quality.
• There is frequently an absence of data on potential confounding
factors if the data was recorded in the past.
• It may be difficult to identify an appropriate exposed cohort and an
appropriate comparison group.
• Differential losses to follow up can also bias retrospective cohort
studies.

25. SELECTION BIAS RETROSPECTIVE COHORT STUDIES
• Bias can occur in retrospective cohort studies if subjects in one of the
exposure groups are more or less likely to be selected if they had the
outcome of interest.
Ex: Retrospective cohort study with the aim of measuring the association
between exposure to an industrial solvent and risk of death.
• study was conducted using employee health records for the past 20 years
from a large company.
• Some of the employees were exposed to the solvent, but others were not.
• During this span of time, some employee health records were lost, but they
were more likely to retain the records of employees who had been exposed
to the solvent and then subsequently died.

26. real risk ratio would
have been found to
be 2.0, if all records
had been retained.
as a result of selective retention of
records for exposed workers who died
prematurely, the apparent risk ratio
was 2.42, i.e. an overestimate of the
association.

27. OUTBREAK OF GIARDIA
• Occurred in Milton
• The request for assistance was made some time after the start of the outbreak,
and the outbreak was winding down by the time DPH began their study.
• The outbreak was clearly concentrated among members of the Wollaston Golf
Club in Milton, MA , which had two swimming pools, one for adults and a
wading pool for infants and small children.
• The investigators thought that contamination of the kiddy pool by a child
shedding Giardia into their stool was the most likely source.
• Investigators knew the denominators for the exposure groups, so they could
calculate the cumulative incidence, risk difference, and the risk ratio.
• People who had spent time in the kiddy pool had 9.0 more cases per 100
persons than those who spent time in the kiddy pool.
• The risk ratio was 3.27.

28. THANK YOU