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

on dependent variable.

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 distribution\trends 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

incidence\prevalence 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

and provides information about the

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

case comparison design.

This approach have helped in identification of

causative factors of many diseases\problems 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

exposed to the risk factors (control group).

Both the groups are followed up forseveral years and observation are madewith reference to frequency anddistribution of the suspected disease (inthis example-lung cancer) over a period oftime. The data is statistically analysed andcomparisons are done between theincidence among smokers and non-smokers to determine the association ofrisk factors to the disease

The prospective study is expensive andtime consuming but it has its advantagesover 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 andidentifies the cases from existing record to be included in thestudy group.

He then traces them from past fixed date forward to presenttime or any other known date i.e. from January 1975 to dateor may be up to December 1995.

Retrospective cohort study designs have been most useful fordetermining the effect of occupation hazards. It is because therecord is easily available e.g. study of lung cancer in uraniumminers. To conduct cohort studies there are three essentialrequisities:

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

controlled conditions.

The investigator administers an

interventions\gives treatment to the

experimental group which is either

exposure to causative factor of disease or

preventive\therapeutic 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. Communitytrials are very expensive and areusually not undertaken unlessessentially required

Community health nurse has manyopportunities in the community settings toconduct experimental studies to determinethe effectiveness of community healthnursing 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.

Thank

you