1. CHAPTER 4
EPIDEMIOLOGY
Qassim Awale MPH,HN&D

2. DEFINITION
• Epi- among, upon;
• Demos = people, population;
• Logos = science, study.

3. DEFINITION
• Epidemiology has been defined as, ‘The
study of distribution and determinants of
health related states or events in specified
populations and application of this study
to the control of health problems’ (John M
Last, 1988).

4. EVENTS
• The health related events are all the
conditions of the spectrum of health such
as disease, injury, disability and death.
• These events are with reference to the
human population.

5. DISTRIBUTION
• This refers to the pattern of occurrence of
disease in the community with reference
to time, place and person.
• This helps to study the trend of the
disease over the years (decades),
geographical areas and over different
population groups.

6. CONTINUE
• This study also helps to know the magnitude
of the problem, gives a clue about the
etiology, mode of transmission of the
disease.

7. DETERMINANTS
• This refers to the etiological or risk factors
related to particular disease. This study helps
to test the etiological hypothesis formulated.
• This aspect of epidemiology dealing with
testing the hypothesis is known as
‘Analytical epidemiology.

8. DISEASE FREQUENCY
• This means measuring the magnitude or
extent of the health related event or health
problem in the community, in terms of
morbidity rates such as incidence and
prevalence and also in mortality rates. These
are expressed in terms of rate, ratio and
proportion. This helps to compare with that
of other countries or other groups of
population in the same country (Morbidity
means sickness and Mortality means deaths).

9. OBJECTIVES OF EPIDEMIOLOGY
• To know the distribution of the disease in
the community.
• To know the magnitude of the problem
• To identify the etiological and risk factors in
the development of disease

10. CONTINUE
• To plan for the implementation of
prevention and control measures
• To eliminate or eradicate the disease
• To evaluate the control measures
• Ultimate objective is to promote the health
and wellbeing of the people.

11. EPIDEMIOLOGICAL APPROACH
• It is an approach to achieve the above objectives by
collecting the data by asking the following questions
and analyzing the data systematically:
• What is the event? (Nature of the disease)
• When did the disease occur? (Time distribution of the
disease)
• Where did the disease occur? (Place distribution of the
disease)
• Who are the persons affected? (Person distribution of the
disease)
• What is the extent of the problem? (Magnitude)
• Why did it occur? (Etiology)
• What is to be done to reduce the problem? (Control
measures)
• How can it be prevented in future? (Preventive measures)

12. Measurements in Epidemiology
• The magnitude of the health problem in the
community is measured in terms of
diseases (morbidity) and deaths (mortality).
• It is expressed in terms of rates, ratios and
proportions, which are called ‘Basic tools’
in epidemiology.

13. Rate
• It measures the occurrence of an event
(disease or death) in a given population,
during a given period of time, usually one
year, e.g. death rate, birth rate, growth rate,
accident rate, etc.
• Death rate is the rate at which the people are
dying in a given area, during a given period
of time.

14. Continue
• for example, out of 500 juveniles in a
juvenile home, while 200 are found to be
delinquents.

15. RATIO
• This expresses a relation between the two
quantities. The numerator is not a
component of the denominator. It is
expressed as x/y or x: y.
• In the above example, the juvenile
delinquency ratio is 200: 300, i.e. 2 : 3.

16. MEASUREMENT OF MORBIDITY
• This means determining the quantum of disease in the
community (i.e. disease load).
• When the survey is carried out over a period of time
(minimum one year) it is called longitudinal survey or
incidence survey, which helps to find out the occurrence of
NEW cases, of a specified disease.
• When the survey is carried out at a given point of time or a
particular period, it is called cross sectional or horizontal or
prevalence survey, which helps to find out the existence of
both OLD and NEW cases in total, of a specified disease.
• Thus, morbidity is measured and expressed in two ways—
incidence and prevalence rates.
• (Incidence is obtained from longitudinal studies and
prevalence from cross-sectional studies).

17. INCIDENCES
• Incidence rate number of new cases of a
specified disease in a given area during a
given period of time.
• It is the occurrence of only NEW (fresh) cases
of a specified disease in a given area, during
a given period of time (minimum one year).

18. PREVALENCE
• It refers to the existence of all cases (both old
and new) of a specified disease at a given
point (point prevalence) or a given period of
time (period prevalence), in a given
population.
• The point of time may be hour, day or week.
The period of time may be months or years.
Period prevalence is the sum of point
prevalence, at the beginning of the specified
period and the incidence during that period

19. Differences B/W Prevalence &
Incidence
Incidence Prevalence
Refers to occurrence of new
cases only in a given area,
during a given year.
Refers to existence of both old and
new cases in a given area, during a
given period/point of time.
Refers to acute cases having
short incubation period.
Prevalence rate is useful to know the
magnitude of the problem
Not influenced by duration
of illness.
Prevalence is not an ideal measure
to study the etiology of a disease.
Incidence rate is an ideal
measure to study the etiology
of a disease
Refers to chronic cases having long
incubation period
Incidence rate helps to
evaluate the control measures
Prevalence rate helps for health
planning.
Examples, measles Ex: Tuberculosis, leprosy, etc

20. EPIDEMIOLOGICAL STUDIES
(METHODS)
• There are two types epidemiological studies
1. Observational.
2. Experimental.
• OBSERVATIONAL STUDIES: are based on
the field observations made on the
experiments carried out by nature. They are
of two types namely
A. descriptive
B. analytical studies.

21. A. Descriptive Studies
(Descriptive Epidemiology)
• These are concerned with observation of the
distribution of a disease in a community,
with reference to time, place and person, and
identifying the associated characteristics of
the disease to formulate an etiological
hypothesis.

22. PROCEDURES
• Defining the population of the community
• Defining the disease under study
• Describing the distribution disease with
reference to time, place and person
• Measurement of disease
• Formulation of etiological hypothesis.

23. TIME DISTRIBUTION
• This means describing the time of
occurrence/onset of the disease with
reference to year, month, week, day, hour of
onset, season, atmospheric temperature,
climate etc. This study often gives a clue
about the etiology of the disease or the
predisposing factors, so that preventive
measures can be adopted.

24. PLACE DISTRIBUTION
• This means the pattern of occurrence of a
disease in different places.
• This helps to compare the disease occurrence
from one country to another country, and
within the same country from one state to
another state, from rural to urban areas and
local areas.

25. PERSON DISTRIBUTION
• This means describing the distribution of a
disease in the community with reference
to the host characters of the persons
affected, such as age, sex, occupation,
literacy level, marital status, social class,
behavior, and such other factors.

26. B. ANALYTICAL STUDIES
(ANALYTICAL EPIDEMIOLOGY)
• This is also an observational study of
epidemiology which deals with testing the
etiological hypothesis, formulated.

27. Uses of Descriptive Epidemiology
• It helps to know the extent/magnitude of the
disease in the community, in terms of morbidity
and mortality rates.
• It helps to know the distribution of the disease
with reference to time, place and person.
• It helps to identify the risk group.
• It helps to formulate an etiological hypothesis.
• It helps to plan, organize and implement
curative and preventive services.
• It helps in doing research.

28. Experimental Epidemiology
(Experimental Epidemiological
Studies)
• Experimental studies are also called
‘Intervention studies’. In this study, some
action or intervention is involved such as
deliberate application (of a drug) or
withdrawal of the suspected cause in the
experimental (or study) group and no change
in the control group.
• Later the outcome of the experiment is
compared in both the groups.

29. CONTINUE
• it differs from the observational studies in
that the experiment is directly under the
control of the investigator and involves
cost, ethics and feasibility whereas in the
observational studies the investigator
(epidemiologist) takes no action but only
observes the outcome.

30. USES OF EPIDEMIOLOGY
• 1. It helps to study the natural history of a
disease, i.e. in relation to agent, host and
environmental factors and further evolution
of the disease to its termination as death or
recovery, in the absence of prevention or
treatment. This is a necessary framework for
application of preventive measures.

31. CONTINUE
• 2. It helps to measure the disease frequency
in terms of the magnitude of the problem
(i.e. morbidity and mortality rates).
• 3. It helps to make ‘Community diagnosis’
by studying the distribution of the disease
with reference to time, place and person.

32. CONTINUE
• 4. It helps to make researches in
epidemiology.
• 5. It helps to identify the determinants of the
disease and the risk factors.
6. It helps to ‘evaluate’ the health services to
find out whether the measures undertaken
are effective in controlling the disease or not.
Further it also helps to find out the cost-
effectiveness of different methods.

33. SL
N
O
EPIDEMIOLOGY CLINICAL MEDICINE
1. Unit of study defined
population.
Individual/case.
2. Relevant data, identifies sources
of infection, mode of spread,
determines future trends &
control measures.
Case, diagnosis, prognosis,
prescribes treatment.
3. Concerned with those who are
sick & those who are healthy
Often concerned with those
who are sick.
4. Investigator goes to community Patients comes to doctor
5. Conceptual, results in the form
of tables & graphs.
Based on biomedical concepts,
refining tech..of diagnosis &
treatment.

36. •THE END