This document discusses different types of observational studies, including cohort studies, case-control studies, and cross-sectional studies. Cohort studies follow groups of individuals over time to determine outcomes. Case-control studies compare individuals with a disease (cases) to those without (controls) to identify potential risk factors. Cross-sectional studies collect data on exposure and outcome at the same time to determine prevalence. While each study type has advantages and disadvantages, together they help identify disease risk factors and generate hypotheses for further research.

1. OBSERVATIONAL STUDIES (COHORT AND CASE-CONTROL STUDIES)
PRESENTATION
BY
BELLO LAWAL DANCHADI
AT
CENTRE FOR ADVANCED MEDICAL RESEARCH AND TRAINING, (CAMRET).
USMANU DANFODIYO UNIVERSITY, SOKOTO.

2. Outline
• Introduction
• Observational studies
• Cohort studies
• Case-control studies
• Cross sectional studies
• Advantages and disadvantages each

3. Introduction
Observational studies
• Also called epidemiologic studies,
• It is a study that is designed to study the following:
• Etiology
• Natural history
• Treatment of rare conditions
• Instances where randomized controlled trial might be unethical, (mann, 2003).
• The researcher does not intervene but rather “observes”, (Concato, et al., 2000).
• Types are: Cohort, Case-conrtol and Cross-sectional studies.

4. Observational study design
Fig. 1. Temporal design of observational studies
• Cross-sectional studies, a prevalence studies without temporal
dimension.
• Evaluate subjects at one point in time, the present time
• Cohort studies can be retrospective or prospective
• Retrospective, “look back in time” and
• Prospective, “look ahead” to examine causal associations.
• Case control, a retrospective studies
• assess the history of subject for the causal exposure.
(Kevin et al., 2010).

5. Cohort studies: Origin
• “Cohort” has military roots, originate from Latin word “cohors”, (Sung and Chung,
2010).
• First appeared in medical literature in 1930s in an article by epidemiologist W.H. Frost,
(Doll, 2001).
• The modern epidemiologic definition:
• Group of people with defined characteristics followed-up to determine incidence,
causes and outcomes, (Merril and Timmreck, 2004).
• Measure the outcomes in their chronological order,

6. Cohort study cont’d
• Used to distinguish between cause and effect,
• Cohort study is of two types, “Retrospective” and “Prospective”, (Kumar et al., 2014).

7. Steps in cohort study design and Data collection
Study design
• Select study population
• Then select the subject and control group
• Follow-up
• Obtain data on exposure
• Analysis
Data collection
Clinical visit
 Physical examination,
 interviews,
 laboratory assay
Personal interviews
Mailed questionnaires
Medical records

8. Fig. 2. Levels of subject selection. (Adapted from Elwood M. Critical Appraisal of Epidemiological Studies and Clinical Trials. 3rd ed. Oxford: Oxford University Press; 2007:1–570.
COHORT STUDY: SELECTION PROCESSES
criteria

9. Cohort study frame work
Fig. 3. Schematic presentation of cohort study design
(Kumar et al., 2014) : Observational Study Designs in Medical Research

10. Advantages and Disadvantages of cohort study
Advantages
• Calculate the relative risk directly
• Study a range of possible risk factors
• Incidence can be directly calculated
• Hypothesis testing
• Several possible outcomes can be
studied,
• Retrospective is cheap and quick
Disadvantages
• Requires large cohort of patients
• Not suitable for rear diseases
• Very expensive
• Time consuming
• Differential loss to follow up can
introduce bias,
(Kumar et al., 2014)

11. Case-control studies
• Compare two groups retrospectively
• Identify people with disease (case patients), people with out disease (controls) and ask
everyone about past exposures
• Help to investigate the disease etiologies through the detailed histories and examination of
patient, (Paneth et al., 2002).
• Measures exposure and outcome and study association,
• Aim to identify predictors outcome and useful for hypothesis generation.
• Only practicable method for identifying risk factors for rare diseases, (Mann, 2012).

12. Case-control study design
Fig 4: A schematic diagram of a case-control study design.
(Kumar et al., 2014) : Observational Study Designs in Medical Research

13. Advantages and Disadvantages of Case-control Studies
Advantages :-
• Easy to conduct as no follow up is
required.
• Gives faster results.
• Inexpensive.
• Suitable for rare and newly
identified diseases.
• More than one risk factors can be
studied simultaneously.
• Requires a smaller study subjects,
• Generate hypothesis for future
studies, (Kumar et al., 2014).
Disadvantages
• Difficult to find suitable control group
• The study depends upon the history given by the
subject. Hence recall bias can occur.
• Cannot differentiate between causes and
associated factors,
• Incidence is unknown hence can’t calculate the
relative risk (RR) directly, (Kumar et al., 2014).

14. Cross sectional studies
• Used to study the prevalence at a given point in time,
• Group with or without the outcome of interest are used (Mann, 2012).
• Only one group is used and data is collected only once,
• Can study multiple outcomes but do not differentiate between cause and effect.
• Done using questionnaire or interview.
• Formulate a research question and choose a study population (Kevin et al., 2010).

15. Cross sectional studies cont’d
• Decide what variable of the study population relevant to the questions (Mann, 2012)
• Method for contacting subject must be devised and implemented,
• Then the data are collected and analyzed (Mann, 2012).
• Quick, cheap and no follow-up required
• Required fewer resources to run the study,

16. References
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Grimes D.A., Schulz K.F., (2002). Cohort studies: marching towards outcomes. Lancet.;359(9303):341-5.
Kumar R., Khan A.M., Chatterjee P., (2014). Types of observational studies in medical research. Astrocyte;1:154-9.
Kevin C. Chung, M.D., M.S. (2010). Observational Studies: Cohort and Case-Control Studies, Section of Plastic Surgery, 126: 2234,
Mann C.J. (2003). Observational research methods. Research design II: Cohort, cross sectional, and case-control studies. Emerg. Med.
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Merril RM, Timmreck TC. (2006). Introduction to Epidemiology. 4th ed. Sudbury, Mass: Jones and Bartlett Publishers;1–352. 7.
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