INTRODUCTION TO EPIDEMIOLOGY

The word epidemiology comes from the Greek words epi, meaning “on or upon,”
demos, meaning “people,” and logos, meaning “the study of.” Epidemiology
touches many aspects of health sciences. It is used to help define the full clinical
picture of a disease. By describing the occurrence of disease in the community,
epidemiology helps public health practitioners and administrators plan for
allocation of resources. Once needed services are implemented, the
epidemiological approach can help evaluate their function and utility.

KEY TERMS USED IN EPIDEMIOLOGY

 Epidemic or outbreak: disease occurrence among a population that is in

excess of what is expected in a given time and place.
 Cluster: group of cases in a specific time and place that might be more than
expected.
 Endemic: disease or condition present among a population at all times.
 Pandemic: a disease or condition that spreads across regions.
 Rate: number of cases occurring during a specific period; always dependent
on the size of the population during that period.

DEFINITION:

Many definitions have been proposed, but the following definition captures the
underlying principles and the public health spirit of epidemiology.

 According to the University of Washington's Department of Epidemiology,

“epidemiology is the study of the frequency, distribution, and determinants
of disease in human populations.” In other words, epidemiologists study
outbreaks of diseases, the causes, locations, and how various communities
are affected. Then they utilize this information to help prevent further
outbreaks and inform the public of how to maintain or improve their health.
 Epidemiology is the science which concerns itself with the natural history of
disease as it is expressed in groups of person related by some common
factors as age, sex, location or disease in an individual. -American
Epidemiological Society.
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  “Epidemiology is the study of the distribution and determinants of health-
related states or events in specified populations, and the application of this
study to the control of health problems.” -James Last

Deconstructing the definition

 Study: Epidemiology is a scientific discipline, sometimes called “the basic

science of public health.” It has, at its foundation, sound methods of
scientific inquiry.
 Distribution: Epidemiology is concerned with the frequency and pattern of
health events in a population.
 Frequency: includes not only the number of such events in a population, but
also the rate or risk of disease in the population. The rate (number of events
divided by size of the population) is critical to epidemiologists because it
allows valid comparisons across different populations.
 Pattern: refers to the occurrence of health-related events by time, place, and
personal characteristics.
 Person- Who?
 Place- Where?
 Time- When?
 Helps us to understand: Why?
 Determinants: Epidemiology is also used to search for causes and other
factors that influence the occurrence of health-related events.
 Health-related states or events: Originally, epidemiology was concerned
with epidemics of communicable diseases. Then epidemiology was extended
to endemic communicable diseases and non-communicable infectious
diseases. More recently, epidemiologic methods have been applied to
chronic diseases, injuries, birth defects, maternal-child health, occupational
health, and environmental health. Now, even behaviors related to health and
well-being (amount of exercise, seat-belt use, etc.) are recognized as valid
subjects for applying epidemiologic methods. In these lessons we use the
term “disease” to refer to the range of health-related states or events.
 Specified populations: Although epidemiologists and clinicians in clinical
practice are both concerned with disease and the control of disease, they
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 differ greatly in how they view “the patient.” Clinicians are concerned with
the health of an individual; epidemiologists are concerned with the collective
health of the people in a community or other area.
 Application: Epidemiology is more than “the study of.” As a discipline
within public health, epidemiology provides data for directing public health
action. However, using epidemiologic data is an art as well as a science.
Epidemiology also deals with control and prevention of diseases in the
community.

SEVEN USES OF EPIDEMIOLOGY (MORRIS, 1975)

1. To study the history of the health of populations.

2. To diagnose the health of the community.
3. To study the working of health services with a view to their improvement.
4. To estimate from the group experience what are the individual risk on
average of disease and the chances of avoiding them.
5. To identify syndromes by describing the distribution and association of
clinical phenomena in the population.
6. To complete the clinical picture of chronic diseases and describe their
natural history.
7. To search for causes of health and disease.

PURPOSES OF EPIDEMIOLOGY IN PUBLIC HEALTH PRACTICE

1. Discover the agent, host, and environmental factors that affect health
2. Determine the relative importance of causes of illness, disability, and death
3. Identify those segments of the population that have the greatest risk from
specific causes of ill health
4. Evaluate the effectiveness of health programs and services in improving
population health

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EPIDEMIOLOGICAL APPROACH

The epidemiological approach to problem of health and disease is based on two

major foundations:

1. Asking question
2. Making comparison

Asking questions: Epidemiology has been defined as “a means of learning or

asking questions…and getting answers that lead to further questions. For example
the following questions could be asked:

Related to health Events

a. What is the event? (the problem)

b. What is its magnitude?
c. Where did it happen?
d. When did it happen?
e. Who are affected?
f. Why did it happen?

Related to health action

a. What can be done to reduce this problem and its consequences and its
consequences?
b. How can it be prevented in the future?
c. What action should be taken by the community? By the health services, By
other sectors, and Where and for whom these activities be carried out?
d. What resources are required? How will the activities to be organized?
e. What difficulties may arise, and how might they be overcome? Answer to
the above questions may provide clues to disease aetiology, and help for
planning and evaluation.

Making comparisons

The basic approach in epidemiology is to make comparisons and draw inferences.

This may be comparison of two (or more groups) – one group having the disease

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(or exposed to risk factor) and the other group(s) not having the disease (or not
exposed to risk factor), or comparison between individuals.

By making comparisons, the epidemiologists’ tries to find out the crucial

differences in the host and environmental factors between those affected and not
affected. In shorts the epidemiologist weighs, balances and contrasts. Clues to
aetiology come from such comparisons.

TYPES OF EPIDEMIOLOGICAL STUDIES

There are two main types of epidemiological study; the Observational and
Experimental Epidemiology/study

1. Observational Studies
a. Descriptive epidemiology
b. Analytical epidemiology
 Case- control
 Cohort study
2. Experimental studies intervention studies
a. Randomized control trials or clinical trials (Most used)
b. Field trials or community intervention studies
c. Community trials
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Observational studies/epidemiology

Descriptive epidemiology / studies are usually the first phase of an epidemiological

investigation. These studies are concerned with observing the distribution of
disease or health related characteristics in human populations and identifying the
characteristics with which the disease in question seems to be associated. In
descriptive epidemiology, we organize and summarize data according to time,
place, and person. These three characteristics are sometimes called the
epidemiologic variables. Compiling and analyzing data by time, place, and person
is desirable for several reasons.

 First, the investigator becomes intimately familiar with the data and with the
extent of the public health problem being investigated.
 Second, this provides a detailed description of the health of a population that
is easily communicated.
 Third, such analysis identifies the populations that are at greatest risk of
acquiring a particular disease. This information provides important clues to
the causes of the disease, and these clues can be turned into testable
hypotheses.

PROCEDURES IN DESCRIPTIVE STUDIES

1. Defining the population to be studied

2. Defining the disease under study
3. Describe the disease by Time, Place & Person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of an aetiological hypothesis

USES OF DESCRIPTIVE EPIDEMIOLOGY/ STUDIES:

i. Provide data regarding the magnitude of the disease load and types of
disease problems in the community in terms of morbidity and mortality rates
and ratios.

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 ii. Provide clues to disease aetiology and help in the formulation of an
aetiological hypothesis. That is, the existence of possible casual associations
between a factor and a disease is usually recognized in descriptive studies.
Thus, if the disease is observed to be more frequent in a particular group
than in others, hypotheses are formulated to explain the increased frequency.
iii. Provide background data for planning, organizing and evaluating preventive
and curative services, and they contribute to research by describing
variations in disease occurrence by time, place and person.

Analytical Epidemiology:

Analytical studies are the second major type of epidemiological studies. In contrast
to descriptive studies that look at entire population, in analytical studies, the
subject of interest is the individual within the population; the object is not to
formulate, but to test hypotheses. Nevertheless, although individuals are evaluated
in analytical studies, the inference is not to individuals, but to the populations from
which they are selected. This study comprises two distinct types of observation
studies;

a. Case control study

b. Cohort study From each of these study designs, one can determine;
 Whether or not a statistical association exists between a disease and a
suspected factor
 If one exists, the strength of the association.

Case control study: Case control studies, often called “retrospective studies” are a
common first approach to test casual hypothesis. In a case-control study, we enroll
a group of people with disease (“cases”) and a group without disease (“controls”)
and compare their patterns of previous exposures. The key in a case-control study
is to identify an appropriate control, or comparison, group, because it provides our
measure of the expected amount of exposure. In recent years, the case control
approach has emerged as a permanent method of epidemiological investigation.
This method has three distinct features;

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 1. Both exposure and outcome (disease) have occurred before the start of the
study.
2. The study proceeds backwards from effect to cause, and
3. It uses a control or comparison group to support or refute an inference.

 Case control studies are basically comparison studies. Cases and controls
must be comparable with respect to known “confounding factors” such as
age, sex, occupation, social status, etc.
 The questions asked relate to personal characteristics and antecedent
exposures which may be responsible for the conditions studied. For
example, one can use as “cases” the immunized children and use as
“controls” un-immunized children and look for factors of interest in their
past histories.
 Case control studies have been used effectively for studies of many cancers,
and other serious conditions such as cirrhosis of the liver, congestive heart
failure, etc.

There are four basic steps on conducting a case control study:

1. Selection of cases and controls

2. Matching
3. Measurement of exposure, and
4. Analysis and interpretation

ADVANTAGES AND DISADVANTAGES OF CASE CONTROL STUDIES

 Advantages:
a. Relatively easy to carry out
b. Rapid and inexpensive (compared with cohort studies)
c. Require comparatively few subjects
d. Particularly suitable to investigate rare diseases or disease about which a
little is known
e. No risk to subjects

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f. Allows the study of several different aetiological factors (e.g. smoking,
physical activity and personal characteristics in myocardial infarction.)
g. Risk factors can be identified. Rational prevention and control programmes
can be established.
h. No attrition problems, because case control studies do not require follow-up
of individuals into the future.
i. Ethical problems minimal.

 Disadvantages
a. Problems of bias relies on memory or past records, the accuracy of which
may be uncertain, validation of information obtained is difficult or
sometimes impossible.
b. Selection of an appropriate control group may be difficult.
c. We cannot measure incidence and can only estimate the relative risk.
d. Do not distinguish between causes and associated factors.
e. No suited to the evaluation of therapy or prophylaxis of disease.
f. Another major concern is the representativeness of cases and controls