“The study of the distribution and determinants of health-related states or events in specified population and the application of the study to control of health problems.”

1. EPIDEMIOLOGY
Harsh Rastogi
PhD student
Community Ophthalmology
Advanced Eye Centre
PGIMER, Chandigarh

2. INTRODUCTION
• The term epidemiology is derived from the Greek word epidemic.
• Epi – among, upon
• Demos- people
• Logos – scientific study

3. DEFINITION
• The widely accepted definition of epidemiology is:
“The study of the distribution and determinants of health related states or events in
specified population and the application of the study to control of health problems.”
(J. M. Last 1988)

4. PRINCIPLES OF EPIDEMIOLOGY
DISTRIBUTION:
Epidemiology studies the frequency and pattern of health events in a given
population.
FREQUENCY:
Epidemiology is measurement of frequency of disease, disability or death.
Thus the basic measure of disease frequency is rates and ratio.

5. PRINCIPLES (CONT.)
PATTERN: Refers to the occurrence of health related events by time, place and
person.
Time- It may be annual, seasonal, weekly, daily, hourly, or any other breakdown
of time that may influence disease or injury occurrence.
Place- This includes the variations found in geographic region, urban and rural
differences, and location of work sites.
Person- It includes demographic factors such as age, sex, marital status,
socioeconomic status, behaviors and environmental exposures.

6. PRINCIPLES (CONT.)
DETERMINANTS:
Determinants are any factor that decides the change in an event, characteristics, or
other definable entity that contributes to the development of some illness.
HEALTH RELATED STATES OR EVENTS:
The term health related states or events may be seen as anything that affects the
well being of a population.

7. PRINCIPLES (CONT.)
SPECIFIED POPULATIONS:
The physician in the hospital shows interest on the health of an individual whereas
the epidemiologist is concerned about the health of the people in a community or
population.
The physician’s focus is the individual patient; the epidemiologist’s “patient” is the
community.
The clinician usually focuses on curing and caring for individual, the epidemiologist
focuses on identifying the exposure or source that caused the illness.

8. PRINCIPLE (CONT.)
APPLICATION:
The significant feature of epidemiology is application of the gained knowledge for
community based practice.

9. CONCEPT OF EPIDEMIOLOGY
Contemporary epidemiology can be divided into four diverse eras, each based on
fundamental casual thinking.
They include:
• Sanitary statistics epidemiology: Theory believed that the disease were caused by
poisonous and malodour elements released from decomposed organic matters.
• Infectious disease epidemiology: Disease etiology is a complex phenomenon
involving host-infectious agent connections.

10. CONCEPT (CONT.)
• Chronic disease epidemiology: Antibiotics and chemotherapy were introduced as
preventative and curative treatments for major infectious diseases following the
identification of their causative agents, but non-communicable diseases with
complex etiologies continue to pose a significant burden.
• Eco epidemiology: In relation to the environment, global health patterns, illness
transmission routes, and disease occurrence are undergoing rapid change.

11. GOALS OF EPIDEMIOLOGY
• To understand the cause of disease variance and apply this knowledge to improve
population and individual health.
• To prevent and control disease, regulate health and health care planning, and
improve individual health care.
• The ultimate objective is to characterize the magnitude and nature of people's
health problems, discover strategies for preventing disease, and improve the
population's health.

12. AIMS OF EPIDEMIOLOGY
• According to the International Association for Epidemiology (IEA), epidemiology has
three primary objectives:
1. To describe the distribution and magnitude of health and disease challenges in
the human population.
2. To uncover etiological factors in disease pathogenesis.
3. Provide the data necessary for the planning, implementation, and evaluation of
services for the prevention, control, and treatment of disease, as well as the
establishment of service priorities.

13. ULTIMATE AIM OF EPIDEMIOLOGY
Epidemiology's ultimate goal is to lead to effective action:
a. To eliminate or minimize health complications or their consequences
b. To enhance the health and well-being of the entire community.

14. PURPOSE OF EPIDEMIOLOGY
• Define and interpret the natural history of diseases.
• Health status of the population is described.
• Identifying disease-causing factors.
• Effectiveness of intervention is evaluated

15. SCOPE OF EPIDEMIOLOGY
• Clinical, biological, social, and ecological disciplines are combined in epidemiology.
• To make a diagnosis, the first step in epidemiological study, epidemiology requires
clinical practice and the sciences.
• Formally, Epidemiology demonstrated an interest in studying and managing
epidemics of communicable diseases.
• Recently, epidemiology has been used to analyze communicable endemic diseases
and noncommunicable diseases.
• Now, epidemiology is a science applicable to all diseases and other health-related
occurrences.

16. USES OF EPIDEMIOLOGY
According to Morris J N the following are the applications of epidemiology.
Morris J N categorized uses into:
• Uses associated with health status and health services
• To investigate the demographic's health's background
• To assess the community's health status
• To study the medical services

17. USES (CONT.)
Applications related to disease etiology:
• To assess individual risks
• To detect syndromes
• To characterize the clinical manifestations of chronic conditions
• To investigate causes of health and disease

18. METHODS OF EPIDEMIOLOGY

19. Types of Epidemiological studies
Non-
experimental/Observational
Studies
Population
based
Descriptive
(Health Survey)
Analytic
(Ecological
Study)
Individual based
Descriptive/
Case reports/
Case series/
Analytic
Cross-sectional
studies/Prevalence
study
Case control
study/Case-
reference
Cohort
study/Follow-up
study
Experimental/In
terventional
Studies
Randomized
Control
Trial/Clinical
Trial
Non-randomized Quasi
experimental/Field
trial/ Community trial

20. INTRODUCTION
Epidemiological studies can be classified as:
1.Observational studies
• Descriptive studies or Health survey
• Analytical studies
• Ecological or co relational
• Cross sectional or prevalence
• Case control or case reference
• Cohort or follow up

21. INTRODUCTION (CONT.)
2. Experimental or interventional studies:
• Randomized control trial or clinical trial
• Quasi-experimental
• Field trials
• Community trials

22. DESCRIPTIVE EPIDEMIOLOGY
• Descriptive studies are frequently the initial phase of an epidemiological
investigation.
• These studies are concerned with observing the distribution of disease or health-
related features in human populations and finding the variables that appear to be
associated with the disease in question.

23. DESCRIPTIVE EPIDEMIOLOGY (CONT.)
Essentially, descriptive studies pose the questions:
• When does the disease manifest? (Time distribution)
• Where is it taking place? (Place distribution)
• Who is infected with the disease?(Person distribution)
Epidemiology
Time
Place
People

24. DESCRIPTIVE EPIDEMIOLOGY (CONT.)
The various procedures involved in descriptive studies are as follows:
• Specify the study population.
• Define the disease being studied.
• Time, place, and person are used to describe a disease.
• Disease characterization
• Using known indices for comparison.
• Etiology hypothesis formulation.

25. DESCRIPTIVE EPIDEMIOLOGY (CONT.)
Specify the
study population
Define the
disease being
studied
Time, place, and
person are used
to describe a
disease
Disease
characterization
Using known
indices for
comparison
Etiology
hypothesis
formulation
The various procedures involved in descriptive studies are as follows:

26. DESCRIPTIVE EPIDEMIOLOGY (CONT.)
• In terms of mortality and morbidity rate, provide information regarding the disease
burden and disease problem in the community.
• Provide a clue to the etiology of the disease and aid in the creation of an etiological
hypothesis.
• Provide background information for service planning, evaluation, organization,
prevention, and treatment.
• They contribute to research by describing variation in disease occurrence by time,
place, and person.

27. ANALYTICAL EPIDEMIOLOGY
• It is the second major type of epidemiological studies.
• In the analytical studies the subject of interest is the individual within the
population.
• The object is not formulated but to test hypothesis.
• It includes two distinct types of observational studies
• Case control
• Cohort study

28. CASE CONTROL STUDY
• It is often called retrospective study, are common first approach to test casual
hypothesis.
• In recent years, the case control approach has emerged as a permanent method of
epidemiological investigation.
• A case control study involves two populations- cases and controls.
• In case control studies the unit is the individual rather than the group.
• The focus is on a disease or some other health problems that has already developed.

29. CASE CONTROL STUDY (CONT.)
The case control method has three distinct features:
• Both exposure and outcome( disease have occurred before start of the study)
• The study proceeds backward from effect to cause.
• It uses the control or comparison group to support or refute an inference.

30. CASE CONTROL STUDY (CONT.)
There are four basic steps in conducting a case control study:
Selection of cases and controls
Matching
Measurement of exposure, and
Analysis and interpretation.

31. DESIGN OF CASE CONTROL STUDY
Direction
of inquiry Time
Population
Cases(
people with
diseases)
Exposed
Not Exposed
Control(
people
without
diseases)
Exposed
Not Exposed

32. COHORT STUDY
• Cohort study is another type of analytical (observational study) which is usually
undertaken to obtain additional evidence to refute or support the existence of an
association between suspected cause and disease.
• This study is also known as prospective study, longitudinal study, incidence study,
and forward looking study.

33. COHORT STUDY (CONT.)
The distinguishing features of cohort studies are:
• The cohorts are identified prior to the appearance of the disease under investigation.
• The study groups, so defined, are observed over a period of time to determine the
frequency of disease among them.
• The study proceeds forward from cause to effect.

34. COHORT STUDY (CONT.)
The three types of cohort studies are as follows:
Prospective cohort study
Retrospective cohort study
A combination of both

35. COHORT STUDY (CONT.)
The elements of cohort study are:
Selection of study subject
Obtaining data on exposure
Selection of comparison group
Follow up and
Analysis

36. EXPERIMENTAL EPIDEMIOLOGY
• In the 1920s, “experimental epidemiology” meant the study of epidemics among the
colonies of experimental animals such as rats and mice.
• In modern usage, experimental epidemiology is often equated with RANDOMISED
CONTROLLED TRIALS.

37. EXPERIMENTAL EPIDEMIOLOGY (CONT.)
• Experimental studies involves some action, intervention or manipulation such as
deliberate application or withdrawal.
• The suspected cause or changing one variable in the causative chain in the
experimental group while making no change in the control group, and observing and
comparing the outcome of the experiment in both the groups.
• They may be conducted in animals or human beings.

38. AIMS OF EXPERIMENTAL STUDIES
• To provide scientific proof of etiological factors which may permit the modification or
control of those diseases.
• To provide a method of measuring the effectiveness and efficiency of health services
for the prevention, control and treatment of disease and improve the health of the
community.

39. RANDOMIZED CONTROL TRIAL
• The basic step in conducting a RCT include the following:
Drawing up a
protocol
Selecting
reference and
experimental
populations
Randomization
Manipulation or
intervention
Follow up
Assessment of
outcomes

41. CONCLUSION
• In conclusion, epidemiology is the study of the health and disease of an entire
population, not a single individual.
• Epidemiology employs a variety of study designs to analyze a specific population.
• This design facilitates precise population studies.

42. Thank You !