This was given as a part of the course of Research & Evidence-Based Medicine given to fourth-year medical students (2018)

1. Research Methodology and
Evidence Based Healthcare (EBHC481)
Cohort Studies
Case-Control Studies
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD1
Lecture 5
Level 8
Year 4

2. Research Methodology and
Evidence Based Healthcare (EBHC481)
Objectives
• Identify cohort design study
• Discuss types of cohort study
• Explain case-control study
• Discuss advantages of case-control study
• Explain difference between retrospective cohort study and a case-
control study
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD2

3. Research Methodology and
Evidence Based Healthcare (EBHC481)
Cohort Study Design
• Cohort studies are a type of medical research used to investigate
the causes of disease, establishing links between risk factors and
health outcomes.
• Cohort studies are usually forward-looking - that is, they are
"prospective" studies, or planned in advance and carried out over
a future period of time.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD3

4. Research Methodology and
Evidence Based Healthcare (EBHC481)
Types of Cohort Studies
• The simplest cohort design is prospective, i.e., following a group
forward in time, but a cohort study can also be 'retrospective'.
• In general, the descriptor, 'prospective' or 'retrospective',
indicates when the cohort is identified relative to the initiation of
the study.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD4

5. Research Methodology and
Evidence Based Healthcare (EBHC481)
1- Prospective cohort
Prospective cohort (may also be called a longitudinal or a concurrent
study)
• An investigator identifies the study population at the beginning of the
study and accompanies the subjects through time.
• In a prospective study, the investigator begins the study at the same
time as the first determination of exposure status of the
cohort. When proposing a prospective cohort study, the investigator
first identifies the characteristics of the group of people he/she wishes
to study.
• The investigator then determines the present case status of individuals,
selecting only non-cases to follow forward in time. Exposure status is
determined at the beginning of the study.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD5

6. Research Methodology and
Evidence Based Healthcare (EBHC481)
Prospective cohort
• Problems: loss to follow up; loss of funding support; continually improving
methods for detecting exposure (leading to greater misclassification than
would be expected in current practice)
• Examples: Nurses Health Study; National Health and Nutrition Examination
Study Follow up Study.
• Exposure was then measured who were followed over a period of time until
reaching the study endpoint.
• A member of the cohort reaches the endpoint either by dying, becoming a
case, or reaching the end of the study period.
• A subject can also be lost to follow-up over the course of the study. The
investigator progresses through time with the subjects in a prospective cohort
study.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD6

7. Research Methodology and
Evidence Based Healthcare (EBHC481)
2- Retrospective cohort study
Retrospective cohort study (historical cohort; non-concurrent
prospective cohort)
• An investigator accesses a historical roster of all exposed and non
exposed persons and then determines their current case/non-case
status.
• The investigator initiates the study when the disease is already
established in the cohort of individuals, long after the original
measurement of exposure.
• Doing a retrospective cohort study requires good data on exposure
status for both cases and non cases at a designated earlier time point.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD7

8. Research Methodology and
Evidence Based Healthcare (EBHC481)
Case-control study
• A case-control study is a type of observational study in which two
existing groups differing in outcome are identified and compared on
the basis of some supposed causal attribute.
• Case-control studies are often used to identify factors that may
contribute to a medical condition by comparing subjects who have that
condition/disease (the "cases") with patients who do not have the
condition/disease but are otherwise similar (the "controls").
• They require fewer resources but provide less evidence for causal
inference than a randomized controlled trial.
• We only get odds ratio from a case control study which is an inferior
measure of strength of association as compared to relative risk.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD8

9. Research Methodology and
Evidence Based Healthcare (EBHC481)
Control group selection
• Controls need not be in good health; inclusion of sick people is sometimes
appropriate, as the control group should represent those at risk of becoming a case.
• Controls should come from the same population as the cases, and their selection
should be independent of the exposures of interest.
• Controls can carry the same disease as the experimental group, but of another
grade/severity, therefore being different from the outcome of interest. However,
because the difference between the cases and the controls will be smaller, this
results in a lower power to detect an exposure effect.
• As with any epidemiological study, greater numbers in the study will increase the
power of the study. Numbers of cases and controls do not have to be equal. In many
situations, it is much easier to recruit controls than to find cases. Increasing the
number of controls above the number of cases, up to a ratio of about 4 to 1, may be
a cost-effective way to improve the study.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD9

10. Research Methodology and
Evidence Based Healthcare (EBHC481)
Selection of cases
• This requires a suitable case definition. In addition, care is needed
that bias does not arise from the way in which cases are selected.
• A study of benign prostatic hypertrophy might be misleading if
cases were identified from hospital admissions and admission to
hospital was influenced not only by the presence and severity of
disease but also by other variables, such as social class.
• In general it is better to use incident rather than prevalent cases.
Prevalence is influenced not only by the risk of developing disease
but also by factors that determine the duration of illness.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD10

11. Research Methodology and
Evidence Based Healthcare (EBHC481)
Advantages of case-control study
• Case-control studies are a relatively inexpensive and frequently
used type of epidemiological study that can be carried out by
small teams or individual researchers in single facilities in a way
that more structured experimental studies often cannot be.
• The case-control study design is often used in the study of rare
diseases or as a preliminary study where little is known about the
association between the risk factor and disease of interest.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD11

12. Research Methodology and
Evidence Based Healthcare (EBHC481)
Advantages of case-control study cont.
• Compared to prospective cohort studies they tend to be less costly
and shorter in duration.
• In several situations they have greater statistical power than
cohort studies, which must often wait for a 'sufficient' number of
disease events to accrue.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD12

13. Research Methodology and
Evidence Based Healthcare (EBHC481)
Examples
• One of the most significant triumphs of the case-control study was
the demonstration of the link between tobacco smoking and lung
cancer,
• Richard Doll and Bradford Hill. They showed a statistically
significant association in a large case-control study. Opponents
argued for many years that this type of study cannot prove
causation, but the eventual results of cohort studies confirmed
the causal link which the case-control studies suggested, and it is
now accepted that tobacco smoking is the cause of about 87% of
all lung cancer mortality in the US.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD13

14. Research Methodology and
Evidence Based Healthcare (EBHC481)
How does a retrospective cohort study differ from
a case-control study?
• Both types of studies identify present cases and non-cases.
• The case-control study identifies the cases and then selects
appropriate controls. An entire cohort is not used.
• If you were investigating an environmentally-related cancer among
university students with a case-control study, you would identify
students within certain years who met the case definition for the
cancer. You would select controls among students who were not a case
of cancer, but matched on characteristics such as age, gender and
graduation year, then determine their exposure status (perhaps
proximity of their campus address to the identified toxin) and compare
exposures between cases and non-cases.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD14

15. Research Methodology and
Evidence Based Healthcare (EBHC481)
Continue …
• A retrospective cohort study uses the entire cohort; all cases and
non-cases within the identified group.
• A retrospective cohort design might designate the cohort to be
students enrolled at the university over a 5 year time span. The
present case status of all these students is determined and
historical data about their exposure status accessed, in order to
assess the relationship between being a case of the cancer with
the exposure.
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD15

16. Research Methodology and
Evidence Based Healthcare (EBHC481)
References
Research methodology manual 2015
Dr. Prabhat Pandey
Dr. Meenu Mishra Pandey
First published, 2015
editor@euacademic.org
ISBN 978-606-93502-7-0
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD16