Final methods epidemiology

METHODS OF
EPIDEMIOLOGICAL
STUDIES
 BY: BHAVISHA PATEL’
LECTURER,
C.M PATEL C.O.N

Three different methods of
epidemiological studies.
1.Descriptive method.
2.Analytical method.
3.Experimental method

 Descriptive and analytical methods or
studies are also known as observational
studies because the investigator does not
intervence.
 He or she makes observation of the
frequency and distribution of the disease by
person, place and time by descriptive and
analyses relationship between health status
and associated factors by analytical studies.
 Experimental studies are intervention
studies. The investigators intervene the
independent variable to determine the effect

 For e.g a study to assess the
effectiveness of PTP on disposal of
excreta related practice of village
people .
 These three methods of
epidemiological studies complement
one another.
 It is very important to define the
population and disease under study to
obtain and interpret data accurately.

Descriptive method
 Descriptive method
cross sectional
studies
Longitudinal
studies

 Descriptive method of epidemiological study is
concerned with the study of frequency and distribution
of disease and health related events in population in
terms of person, place and time.
 Its purpose is to provide an overview of the extent of
health problem and to give clue to possible etiological
factors involved.
 This method gives information about who all are
affected by a particular disease or health related event
or problems, where the cases occur and when they
occur. The data is collected about:

I. Personal characteristics such as
age, race, marital status,
occupation, education, income,
social class, dietary pattern, habit.
II. Place distribution of cases i.e. areas
of high concentration, low
concentration and spotting of cases
in the map.
III. Time distributiontrends such as
year, season, month, week , day
and hour of onset of the disease.

 Such information give clue to possible
associated factors such as age with specific
disease e.g. measles, diphtheria, pertusis in
early childhood, cancer in middle age,
artheroseclorises in old age, some habits like
smoking with lung cancer, dietary pattern with
obesity; seasonal variations such as periodic
fluctuation, consistent time trend. The data
collected are analyzed and presented in terms
of percentage, rates and ratios.

 Thus descriptive epidemiology
provides:
I. Data for describing the nature of disease or problem
and measuring their event in terms of
incidenceprevalence rate, ratios, mortality. By age,
sex, occupation, social class, rates etc. These
information help in making community diagnosis.
II. clues to the etiology of diseases for further rigorous
investigation and confirmation of the causes.
III. Background information for Planning, organizing,
implementing and evaluating preventive, curative
services to deal with these problems.

Types of descriptive studies
 Cross- sectional studies: in this
design of descriptive method of study,
the data is collected from a cross-
section of population at a one point in
time. The results of the study are
applied on the whole population.
 The cross-section of the population is
samples carefully so that it is
representative of the whole population.
cross-section study is like a snapshot

 It is also called as prevalence
study. cross-sectional studies are
useful for detailed community
assessment, study of morbidity and
underlying factors especially
chronic diseases.
 For example study of diabetes or
hypertension by personal
characteristics and life style. These
studies are economical and
comparatively quick to perform

II- LONGITUDINAL STUDIES
 In this design, the data is collected from
the same population repeatedly over a
continuous period of time by follow-up
of contacts and their examination.
 Longitudinal studies are useful for
studying the natural history of diseases,
finding out incidence rates of diseases
and identifying risk factors of disease.
 Longitudinal studies are more
expensive and time consuming than
cross- sectional studies.

2. Analytical method
 In descriptive study we generate etiological clues for various
disease which help in formulating a guess or hypothesis for
further vigorous study or testing e.g. “cigarette smoking(10 to 20
in a day) causes lung cancer in 10 to 15% of smokers after 20
years of exposure”; “wife battering is related to violence in
victim’s childhood family of origin”.
 These types of hypothesis are further studied and tested by
analytical studies to determine the association of cause
with the effect. Thus analytical studies go beyond descriptive
studies.
 Analytical studies are more specific in focus, test hypothesis and
attempt to determine causal factors of disease. Analytical
studies are of two types:

Types of analytical studies
1. Case control study
2. Cohort study

I-Case control study
 In this method a group of people who
have been diagnosed as having a
particular problem e.g. lung cancer
(cases) are compared with a group of
people who are similar in
characteristics to that of cases but
they are free from the problem i.e.
free from lung cancer under study
(control). Here the approach used is
retrospective

Retrospective approach
 i.e. the disease have already occurred and
the epidemiologist looks back over time for
presence or absence of suspected causal
factors in both cases and controls.
 He reviews the records, interviews the cases
and their family members. The data thus
collected about the suspected factors is
analysed stastically to determine the extent
of its association with the disease. This
method , therefore, is called as
retrospective method. It is also called as

 This approach have helped in identification of
causative factors of many diseasesproblems etc.
 Rubella in mothers during early pregnancy is the cause
of congenital deformities in children, smoking
associated with lung cancer, iodine deficiency
associated with hyperthyrodism.
 These conclusions are based on repeated case control
studies. Case-control studies are easy to organize and
are less expensive but very effective in determining the
risk factors.
 The major drawback of this design is difficulty in
selecting the control group.

II- COHORT STUDY
 A Cohort is a specific group of people, at a certain time,
sharing common characteristics or experience e.g. people
born on the same day or the same year (birth cohort), couple
married in the same year (marriage cohort), class of nursing
students (experience cohort), people with same occupation
(occupation cohort) etc.
 cohort studies can be designed in three different ways,
these are:
a) Prospective cohort
b) Retrospective cohort
c) A combination of retrospective and prospective cohort

a. Prospective cohort study
 Cohort study is prospective in nature
because the group under study is free from
the disease but exposed to risk factor and
epidemiologist study the development of a
condition over time. In this method of study
the epidemiologist selects a cohort i.e. a
group of people say in the same age group
and who are exposed to risk factor say
“cigarette smoking (study group) and a
group of people in the same age group and
other similar characteristics but not

 Both the groups are followed up for
several years and observation are made
with reference to frequency and
distribution of the suspected disease (in
this example-lung cancer) over a period of
time. The data is statistically analysed and
comparisons are done between the
incidence among smokers and non-
smokers to determine the association of
risk factors to the disease
 The prospective study is expensive and
time consuming but it has its advantages
over retrospective method

 This method can help in studying the
natural history of disease, estimating
incidence rates, risk of developing
disease.
 It also helps to identify the relationship
of the risk factors under study to other
disease or problems e.g. cigarette
smoking and high blood pressure, cardio-
vascular problems etc.

b. Retrospective cohort study
 This is also called as historical cohort study. In this type of
study designs the event has already occurred.
 The investigator goes back in time – 10 to 30 years and
identifies the cases from existing record to be included in the
study group.
 He then traces them from past fixed date forward to present
time or any other known date i.e. from January 1975 to date
or may be up to December 1995.
 Retrospective cohort study designs have been most useful for
determining the effect of occupation hazards. It is because the
record is easily available e.g. study of lung cancer in uranium
miners. To conduct cohort studies there are three essential
requisities:

 It must be possible to identify from records the
members of some previous existing group e.g.
industrial workers who were exposed to a suspected
cause in the past.
 Information about the factor under study is available
from the record or it can be reconstructed.
 Information about the outcome i.e. disease or death
for the cohort is available.
hospital records can help in designing cohort
studies for determining causal relationship for various
diseases. These studies can be done in a short period
and are economical in comparison to prospective
cohort study.

c. Combination of retrospective and
prospective cohort study
 This design has the characteristics of both
retrospective and prospective studies. The
investigator identifies the cohort from
past record and traces forward till date.
The same cohort is followed up further
for assessment prospectively up to desired
period.

3. Experimental method
 Experimental studies are similar in
approach to cohort studies except that
conditions are under the careful control
of investigator.
 Experimental studies are done to
confirm the etiology of diseases,
establish the efficiency of preventive or
therapeutic measures and evaluate
health care services.
 These studies are done under

 The investigator administers an
interventionsgives treatment to the
experimental group which is either
exposure to causative factor of disease or
preventivetherapeutic measure to
improve or influence health or prevent
disease but not to the control group which
is similar to experimental group in all its
aspects. He then observes and analyses
the outcomes using statistical methods
and confirms the cause of diseases, and
establishes the efficacy of preventive
measures and drugs under study..

Usually these experiments are done in
the laboratory animals.
But clinical and community trials are
done to determine the efficacy of
preventive or therapeutic measures
after the efficacy of these measures
(i.e. vaccines and drugs) is
established on laboratory animals.

These trials do involve medical,
ethical and moral issues. Community
trials are very expensive and are
usually not undertaken unless
essentially required
 Community health nurse has many
opportunities in the community settings to
conduct experimental studies to determine
the effectiveness of community health
nursing practices

 e.g. efficacy of different health education
methods, motivation techniques,
communication methods, self help model
etc.
 some times experiments occur by nature
in the community like occurrence of any
disease which may occur in some areas of
the community and may not occur in
some other areas of the community.

 This is a natural experimental design which
can be studied by community health
personnel to find out why is it so? The most
common example which is often cited is one
which was discovered by John Snow in 1854
in London regarding outbreak of cholera. He
observe that the community contracted the
disease was because of unsafe water supply
and the other community which did not get
cholera did not have the same water supply.

Final methods epidemiology

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Final methods epidemiology