PRINCIPLES OF EPIDEMIOLOGY AND
METHODS OF EPIDEMIOLOGICAL
Submitted To: Submitted By:
Dr. Seep Mahajan Abhishek kashyap
Components of epidemiology
Aims of epidemiology
Principles of epidemiology
Methods of epidemiological studies
Epidemiology is derived from the Greek words,
Epi: means upon.
Demos: mean study of population or People.
Logos: means the scientific study
So, epidemiology is study of disease pattern in human population or study
of effects of multiple factors on human health.
The study of distribution and determinants of health related
states in specified populations, and the application of this study
to control health problems.
Is the basic science of public health.
Provides insight regarding the nature, causes and extent of
Provides information needed to plan and target resources
AIMS OF EPIDEMIOLOGY
1. To identify the etiological factor in disease pathogenesis.
2. To demonstrate the data required for the planning, implementation,
and evaluation of services for disease prevention, control, and
treatment, as well as the establishment of priorities among those
3. It describes the extent and distribution of health disease problems in
the human population
1) To study historically the rise and fall of disease in the population.
2) Community diagnosis.
3) Planning and evaluation.
4) Evaluation of individuals risks and chances.
5) Completing the natural history of disease.
6) Searching for causes and risk factors.
1.)TO STUDY HISTORICALLY THE RISE AND FALL OF DISEASE
IN THE POPULATION
It is well understood that the pattern of health and disease in a
community is never consistent.
There are fluctuations over both short and long time periods.
Epidemiology is the study of disease profiles and time trends in the
We can make useful projections into the future and identify emerging
health problems and their correlates by studying these trends.
Community diagnosis is one of the applications of epidemiology.
The identification and quantification of health problems in a community
in terms of mortality and morbidity rates and ratios is referred to as
It aids in the identification of at-risk individuals or groups, as well as
those in need of health care.
3.)PLANNING AND EVALUATION
It is about the distribution of health problems over time and space,
which provides the fundamental foundation for planning and developing
needed health services, as well as assessing the impact of these services
on people's problems.
Evaluation is an important aspect of epidemiology.
Any measures taken to control or prevent a disease must be followed by
an evaluation to determine whether the measures are effective in
reducing disease frequency.
4.) EVALUATION OF INDIVIDUALS RISKS AND
One of the important tasks of epidemiologists is to make a statement
about the degree of risk in a population.
5.)COMPLETING THE NATURAL HISTORY OF
Epidemiology is concerned with the entire spectrum of disease in the
By studying disease patterns in the community in relation to agent,
host and environmental factors is in better position to fill up the gaps in
the natural history of disease than a clinician.
6.)SEARCHING FOR CAUSES AND RISK FACTORS
Epidemiology helps to identify the causes and risk factors of diseases.
Amount and distribution in populations
person,place and time
Test hypothesis, identify and quantify risk or
➔ Clinical trials
➔ Preventive trials
➔ Case control
➔ Cross sectional
➔ Case reports
➔ Case series
➔ Ecological studies
➔ Cross sectional
To know the situation i.e, what is the problem? , what are its
To describe the general characteristics of disease in relation to time,
Person: who is getting sick?
Place: where is the sickness occurring?
Time: when is the sickness occurring?
1.) CASE REPORT
It consists of a careful and detailed report by one or more clinicians of
unusual medical condition.
It represents first clue in the identification of a new disease.
It leads to formulation of new hypothesis.
Intestinal obstruction was reported in a young child.Documents
showed that this child received ROTA virus vaccine 3 months ago. A
detailed report about this unusual event & exposure was published
in journal. The investigator formulated a hypothesis that rota virus
vaccine may have been responsible for the rare occurrence of this
It is the only study which depend on Routine Surveillance.
8 cases of lung cancer were admitted to different hospitals during
same period of time. Taking history from these patients showed that they
were miners.This unusual circumstance suggested that the miners may
been exposed to something. Investigating this circumstance showed high
concentration of radon gas. A hypothesis was formulated that lung cancer is
related to exposure to randon.
1.) They formulate a new hypothesis for
1.) For case report, the presence
of any exposure may be
coincidental because it is based
on a single experience.
2.) They act as trigger as they
stimulate the start of analytic
studies to be conducted to
identify the risk factors of the
2.)Lack of the comparison group
in case series can either obscure
the relationship or suggest an
association which is not actually
3.) Modification of the case series
to be a case control study can be
obtained by using a comparison
3.)Both of them cannot be used
to show the causal
association,i.e. Can not be used
to test the hypothesis.
The source of data is the entire population.
It compares disease frequencies:-
Between different population during the same period of time or
In the same population at different time.
It compares 2 quantitative variables.
EXAMPLE :- Correlation between one of climatic indicator (Temp.) and
frequency of cerebrovascular storks.
FIG. SHOWS THE CORRELATION BETWEEN THE AVERAGE REGIONAL TEMP. AND
FREQUENCY OF CVSS IN DIFFERENT COUNTRIES. COUNTRIES WITH HIGHEST
AVERAGE TEMP. HAVE HIGHEST RATES OF CVSS AND VICE VERSA.
Advantages & Disadvantages of Ecological Studies
1. Formulates new hypothesis 1.)They cannot be used for
2.) Quick and Cheap 2.) Lack of ability to control
4.)CROSS SECTIONAL STUDY
With exposure &with disease
With exposure & without disease
Without exposure & without
Without exposure & with disease
EXAMPLE:- DURING THE YEAR 2019, A REPRESENTATIVE SAMPLE OF
SECONDARY SCHOOL CHILDREN IN A CITY X (N=400) WERE ASKED ABOUT
CONSUMPTION OF HIGH CALORIC DIET AND EXAMINED TO DETECT OBESITY.
Consumption of high
caloric diet with
Consumption of high caloric
diet without obesity n=60
No consumption of high
caloric diet with obesity
No consumption of high
caloric diet without obesity
ADVANTAGES & DISADVANTAGES OF CROSS-
1.)Formulation of hypothesis 1.)Can’t be used to test
2.)Suitable for chronic diseases
with long latency
2.)Can’t be used in acute
diseases of short duration
3.)Quick and cheap 3.)Not suitable for rare diseases
To identify causal relationships between some risk factors and
occurence of disease.
Try to answer why the disease occurs.
Their types are:-
1.) case control
2.)cohort :- -Prospective & Retrospective
3.) Comparative cross-sectional
OBSERVATIONAL STUDY TYPE
(1.) CASE CONTROL STUDY
The investigator enrolls a group of people without disease (controls).
Investigator then compare previous exposures between the two groups.
The control group provides an estimate of baseline or expected amount
of exposure in that population.
1. Study to determine an association between lung cancer and radon
2. Study to determine association between salmonella infection and
eating at a fast food restaurant.
(2.) COHORT STUDY
Study population is grouped by exposure status.
Groups are then followed to determine if they develop the outcome
EXAMPLE:- study to determine if smokers have a higher risk of
Prospective Assessed at
beginning of study
Followed into the
future for outcome
Retrospective Assessed at some
point in the past
Outcome has already
(A.) PROSPECTIVE COHORT STUDY:- IT IS A LONGITUDINAL
COHORT STUDY THAT FOLLOWS OVER TIME A GROUP SIMILAR
INDIVIDUALS(COHORTS) WHO DIFFER WITH RESPECT TO CERTAIN FACTORS
UNDER STUDY, TO DETERMINE HOW THESE FACTORS AFFECT RATES OF CERTAIN
(B.) RETROSPECTIVE COHORT STUDY:- THESE ARE TYPE OF
OBSERVATIONAL RESEARCH IN WHICH THE INVESTIGATOR LOOKS BACK IN TIME
AT ARCHIVED TO EXAMINE WHETHER THE RISK OF DISEASE WAS DIFFERENT
BETWEEN EXPOSED AND NON-EXPOSED PATIENTS.
A cross-sectional study is defined as a type of observational research
that analysis data of variables collected at one given point in time
across a sample population or a pre-defined subset. This study is also
known as Cross- Sectional Analysis.
EXPERIMENTAL STUDY TYPE
Active trial to change disease determinant by the investigator who
allocates the exposure & follow the subjects.
Can be viewed as a type of prospective cohort study.
ETHICAL POINTS must be considered:
1. It should have beneficial effect to patients, not to harm anyone by
2. Participants should know what the experiment is and have right to
3. If any unplanned complications occur to any participant he should be
excluded from the trial and treated.
(1.) CLINICAL TRIALS:-
It is usually used to assess efficacy of a new line drug or to compare 2
types of drugs.
Diseased subjects are randomly allocated into 2 groups, who are given
“new drug” and “control group” (who are given usual drug or no drug in
Results are assessed by comparing health improvement of the 2 groups
at end of trial.
Example: surgical or medical treatment of peptic ulcer
(2.) COMMUNITY TRIALS:-
Involve people who are not diseased and the sample is drawn from
Data collection takes place in the field.
EXAMPLE:- in studies carried out to assess the efficacy of new vaccine.
The participant are divided into 2 groups: one who is experimental
group ( will take the new vaccine) and 2nd is the control group ( will not
The participant will be followed to compare the level of occurence of the
disease in both groups. Therefore, these groups should be alike as much
as possible in all aspects other than new drug.