3. Descriptive studies
The first step of any epidemiological
investigation
Description of the occurrence of a disease
in a population
Time, Place, Person distribution
Identification of the characteristics with
which the disease under study seems to be
associated.
4. Descriptive studies
Studies of changing pattern of health &
disease over a period of time & space.
Community diagnosis of health problem or
assessment of needs.
Studies of existing data( case series ,
surveillance report, disease registries )
Studies of the natural history of disease
5. Descriptive studies
• Time distribution
When is the
disease occurring?
• Place distribution
Where is it
occurring?
• Person distribution
Who is getting
the disease?
6. Descriptive studies
Purpose - Use information to formulate
Hypothesis ultimately for taking health
action
Unit of the study - populations or its sub
groups, not on individuals
7. Study Instruments
a) Questionnaire & interview schedules
• 1. preparation , pre-coding , pretesting of the
questionnaire
• 2. plan for interviews
• 3. training of interviewers
• 4. Preparation of instructional manual
b) other methods of observation
• Medical examination
• Laboratory tests
• Screening procedures.
Tools with which data is collected
8. Steps in descriptive studies
1. Defining the population to be
studied
2. Defining the disease under the
study
3. Describing the disease by
a) Time b) Place c) Person
4. Measurement of disease
5. Comparing with known indices.
6. Formulation of aetiological hypothesis
9. 1. Defining the population
“Defined population” can be
•whole population, or
•representative sample
•specifically selected group such as age, sex,
occupational groups, school children ,
pregnant mothers etc.
The concept of defined population
is critical because it provides the
denominator for calculating rates.
10. Describing the study
population
• Total Number
• Age composition,
• Sex composition,
• Occupations,
• Socioeconomic
status,
• Literacy profile
• Social customs,
habits
• Specific lifestyles
• knowledge of health
facilities available
and their utilization
11. Vital requirements of study
population
Then only the results of
hypothesis tested on any study
population can be generalized
to the reference population.
1. Its representativeness to the
parent reference population
2. Its optimum size
12. 2. Defining the disease
under study
• The disease or condition can
be identified and measured in
the defined population with a
degree of accuracy.
‘Operational
definition’
This is required so as to enable
observer to identify those who have
the disease from those who do not
have.
13. • ’Tonsillitis’ is defined as an
inflammation of the tonsils, caused by
infection, usually with streptococcus
pyogenes.
Clinical
definition
• Tonsillitis would include the presence of
enlarged, red tonsils with white
exudates which on throat swab culture
grow predominantly s.pyogenes
Operational
definition
Disease definition
14. 3. Describing the disease
Describes the occurrence and distribution of
the disease by
The time
• year, season, month, week, day.
The place
• country,cities,towns,urban/rural.
The persons who are affected with the disease.
• age, sex, occupation, education, S.E.status personal habits,
Ht.,wt., B.P. etc.
15. The Basic Triad of Descriptive
Epidemiology
The three essential characteristics of disease
we look for in descriptive epidemiology:
TIME
PERSONPLACE
16. Time distribution
1. Short term fluctuations(hours, days,
week ) = Epidemics
2. Periodic fluctuation ( months) =
Cyclic trends /seasonal variation
3. Long term or
secular trends
17. Short term fluctuations
Epidemic
Def.- “ The occurrence of cases of an illness
or other health related events in a region or a
community clearly in excess of the normal
expectancy.
Epidemic curve – A graph of the time
distribution of epidemic cases.
It shows time relationship with exposure to a
suspected source.
19. Types of epidemics
A) Common
source
epidemics
•a) Single exposure
or ‘point source’
epidemics
•b) Continuous or
multiple exposure
epidemics
B)Propagated
epidemics
• person to person
• arthropod vector
• animal reservoir
C) Slow
(modern)
epidemics
20. Point source epidemic
There is sudden rise and sudden fall
There are no secondary curves.
Large number of cases occur with a
narrow interval of time
All cases have the same incubation period.
Exposure is almost simultaneous and brief.
All cases develop almost simultaneously following single
exposure. e.g. food poisoning, Bhopal gas tragedy.
Characteristics -
21. 1. Point Epidemic
Short-term changes
occur over limited
time frames
• Hours
• Days
• Weeks
• Months
Used for short-term exposures or diseases with short
incubation and/or illness durations
24. Continuous exposure epidemic
• Epidemic occurs from common source.
• Epidemic is not explosive.
• Exposure occurs continuously or repeatedly not
necessarily simultaneously.
• Sudden rise & gradual fall of the curve.
• e.g. CSW as a source of gonorrhea, infecting all her clients
over period of time.
• Well with contaminated water in outbreak of cholera.
26. Propagated epidemics
A gradual rise & tails off over a
long period of time.
Transmission continues till
susceptible individuals are
exposed to infected persons.
speed of spread depends
upon Herd immunity
The propagated epidemics are most often of infectious
origin usually results from person to person transmission
of infectious agent.
e.g. Hepatitis A , poliomyelitis, Measles
Characteristics :
27. Propagated Epidemic
Initial Period of
Epidemic
Height of
Epidemic
Termination of
Epidemic
O O O O O
O O O O O
O O O O O O O O
O O O O O
O O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O
O O O O O O
O O O O O O O
O O O O
O O
Susceptible Immune
O Fail to infect othersO Infects others
29. 2.Periodic fluctuations
Periodic fluctuations
Seasonal variations Cyclic trends
Seasonal trends:
It is a prominent feature of infections.
for e.g.-
Measles and chickenpox
in the early spring seasons.
URTI
in winter season ,
Diarrheal diseases
during summer months.
Cyclic trends :
certain diseases appear in cycles
which may be spread over short
periods of time like
days, weeks, months or years.
e.g.- epidemic of
Measles in every 2-3 yrs,
Influenza once in 7 to 10 yrs.
Accidents more on week ends.
32. 3.Long term fluctuations
Changes occur over long period of time.
(usually > 10yrs).
It could be increasing or decreasing, real or apparent,
communicable diseases or non-communicable disease.
• e.g.. 1) downward trend : Plague & cholera.
2) upward trend : DM, CHD, lung cancer
Secular trends
33. Why we should know
time trends?
• To know diseases which are increasing or
decreasing & emerging health problems.
• Can frame effective measures to control the
diseases.
• Formulate etiological hypothesis.
• Provide guidelines to health administrator in
matters of prevention or control of disease.
34. Place Distribution
knowledge of geographic pattern of
diseases are major important sources of
clues about the etiology of disease.
International
variations
National
variations
Rural–urban
variations
Local
distributions
35. 1)
International
• a) Malaria,
Leprosy in hot
and humid
climate. e.g..
Africa and
South America.
• b) Ca stomach
– Japan, Ca
oral cavity, Ca
cervix – India.
2)
National
• Endemic
goitre,
lathyrism,
malaria ,filaria
leprosy etc.
3)
Local
• Studied with
the help of
spot map in a
given area.
• E.g.. a) John
Snow study on
cholera in
London.
• b) Endemic
flurosis in
Nalgonda,
Kolar
4)
Urban - Rural
• Tetanus, OP
poisoning,
zoonotic
diseases
common in
rural.
• RTA, drug
abuse, DM
cancer,
mental stress
more
common in
urban .
Place distribution
38. Spot map
It is a graphical presentation of the place
distribution of the disease of occurrence.
“ clustering “ of cases suggest common source of
infection & mode of spread.
e.g. Investigation of cholera epidemic by John
Snow of England with help of spot map
Factors influencing geographical variations are
culture, standards of living, external environment
and genetic factors.
39. 1) Age
2) Race, religion &
ethnicity
3) Gender : Male,
Female.
4) Occupation :
agricultural /
Industry.
5) Marriage : Single,
married, divorce,
separated.
6) Residence.
7) Socio-Cultural
environment.
8) Socio-Economic
background.
9) Behavior ( lifestyle)
10) Stress
11) Migration
Person Distribution
41. Person distribution
Age – Bimodality
Rateperlakhpopulation
0 8070605040302010
7
6
5
4
3
2
0
1
90
Bimodality of Hodgkin’s disease distribution
42. 4. Measurement of disease
measurement of mortality
measurement of morbidity
• Incidence ( longitudinal study) &
• Prevalence ( cross-sectional study)
‘Disease load’ in population
43. 5. Comparing with known
indices
By making comparison between
• Different populations or
• Subgroups of the same population
1.It is possible to arrive at clues to disease
aetiology.
2. Identify groups which are at ‘high risk’ for the
disease.
44. 6. Formulation of hypothesis
e.g. The smoking of 30-40 cigarettes per
day causes lung cancer in 10% of smokers
after 20 years of exposures.
• Population ( characteristics of persons )
• The specific cause
• The expected outcome – disease
• The dose- response relationship
• Time- response relationship
An epidemiological hypothesis should specify
45. Descriptive studies
Case report Case series
KAPB /
opinion
study
Ecological /
co-relation
study
Cross
sectional
study
Longitudinal
study
46. Cross sectional study
Disease frequency survey or
‘ Prevalence study’
• Simple descriptive epidemiological study.
• Single examination of all subjects in population or
re-presentative sample for presence or absence of
disease or risk factors at one point of time.
• Field based , suited for chronic diseases
• Useful for screening of population for undiagnosed
disease.
47. Cross sectional study
Data collected – age, sex, family history, physical
exercise, body weight, salt intake, lifestyle etc.
Study tells aetiology & distribution of disease in
population
Multi-factorial causation
Help in prevention of disease by possible interventions.
Example : Study the prevalence of hypertension
48. Cross sectional study
Uses
• 1. Determine the prevalence of disease.
• 2. Identify possible causative factors in disease.
• 3. Study shows association between variables
but they do not establish causality.
• 4. Use to formulate hypothesis
49. Longitudinal study
Incidence study
Observations are repeated in the same population over
a prolonged period of time by follow up examination.
Useful to study natural history of disease.
Helps to find out incidence rate.
Helps to identify risk factors of disease.
study is time consuming & costly.
Dropouts during study period ( Attrition)
50. Differences between longitudinal & cross
sectional study
LONGITUDINAL STUDY CROSS SECTIONAL STUDY
Observations are repeated by means
of follow-up exam. in population
Observations are done only once in
the population
Carried over a long period of time
( minimum one year)
Carried over a given point of time
This help to find out the occurrence
of new cases (incidence)
This help to find out the existence
of both old & new cases
(prevalence)
Helps to study the natural history of
the disease & the risk factors
Does not helps to study the natural
history of the disease & the risk
factors
Study is time consuming , difficult &
costly
Not time consuming , easy & cheap
51. Uses of
Descriptive epidemiology
Helps to know magnitude of disease in community.
• morbidity & mortality rates
Helps to know the distribution of the disease.
• time, place, person
Helps to formulate an etiological hypothesis.
Helps to plan, organize & implement curative and preventive services.
Helps in doing research.
