The document discusses descriptive methods in epidemiology, highlighting the definition, history, and types of descriptive studies, along with their characteristics, merits, and limitations. It emphasizes the importance of descriptive studies as preliminary investigations that describe health status within populations, focusing on person, place, and time variables. The document also covers the various study designs and concludes that descriptive studies are vital for understanding disease distribution and aiding in further analytical research.

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DESCRIPTIVE METHODS
IN EPIDEMIOLOGY
PRESENTER :DR.AKILA.G.V
GUIDE :DR.SUMAN.G

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CONTENTS
 Introduction
 History of epidemiology
 Definition of epidemiology
 Study designs in epidemiology
 Characteristics of descriptive study
 Descriptive triad pentad

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 Types of descriptive study
 Merits and demerits
 Uses
 Conclusion
 References

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INTRODUCTION
 A descriptive study is the simple description of the
health status of a community and is often the first step
in an epidemiological investigation.
 The first foray into a new disease or area of inquiry—
the first scientific “toe in the water”
 “Concerned with and designed only to describe the
existing distribution of variables, without regard to
causal or other hypotheses.”

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HISTORY OF EPIDEMIOLOLGY
 Hippocrates -“On Airs,
Waters, and Places”
 John Graunt-Natural and
political observation on the
bills of mortality(1662)

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 William Farr-the father of
modern vital statistics and
surveillance –vital statistics
(1885)
 John Snow -“the father of
field epidemiology.”

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DEFINITION OF EPIDEMIOLOGY
 Epidemiology is the study of the distribution and determinants
of health-related states or events in specified populations, and
the application of this study to the control of health problems.
 Distribution. Epidemiology is concerned with the frequency and
pattern of health events in a population. Also called descriptive
epidemiology.

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 Determinants. Epidemiology is also used to search for causes
and other factors that influence the occurrence of health-related
events
 Health-related states or events. Epidemiologic methods have
been applied to chronic diseases, injuries, birth defects,
maternal-child health, occupational health, and environmental
health,amount of exercise, seat-belt use, etc.

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STUDY DESIGNS IN EPIDEMIOLOGY
 Descriptive studies
Populations(correlation studies)
Individuals
Case reports
Case series
Cross sectional studies
 Analytical studies
Case contol studies
Cohort studies
 Interventional studies

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CHARACTERISTIC OF DESCRIPTIVE STUDY
A descriptive study can be defined as one in which only one
group, i.e. subjects having the outcome (disease or any other
health related phenomena of interest) are studied, without
any comparison group, for describing the outcome or health-
related phenomena according to its frequency or such other
summary figures (as mean), and its distribution according to
selected variables related to person, place and time.

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 They do not proceed to test a “pre - formed hypothesis”
 The study is done only on one group of subjects; there is
no comparison group.
 The main objective of a descriptive study is to “describe
”the “mean value” of a health related condition, or a
“proportion” or the natural history of disease or a health -
related phenomena

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 The study also intends to describe the “distribution”
according to variables related to “person” “Place” and
“time” .
 A descriptive study culminates by making suggestions /
hypotheses about certain “cause and effect” relationships
which can be further tested by analytical studies.

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DESCRIPTIVE TRIAD ----- PENTAD
Traditional descriptive epidemiology has focused on three key
features: person, place, and time and should also answer five
basic “W” questions—who, what, why, when, and where
 Who has the disease in question? ? Age and sex are
universally described. Eg venous embolism,leiomyomas

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 What is the condition or disease being studied? Development of
a clear, specific, and measurable case definition is an essential
step in descriptive epidemiology. Generally, stringent criteria for
case definitions are desirable.
 Why did the condition or disease arise? Descriptive studies often
provide clues about cause that can be pursued with more
sophisticated research designs

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 When is the condition common or rare? Time provides
important clues about health events. E.g the outbreak of
gastroenteritis soon after ingestion of staphylococcal toxin
 Where does or does not the disease or condition arise?
Geography has had a huge effect on health. Living close to
rodents and insects (and thus their parasites.Latitude plays a
part in both multiple sclerosis and vitamin D deficiency.

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TRIAD
DISTRIBUTION ACCORDING TO TIME

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SHORT-TERM FLUCTUATIONS
 The best known short-term fluctuation in the occurrence of a
disease is an epidemic. According to modern
concepts ,epidemic is defined as "the occurrence in a
community or region of cases of an illness or other health-
related events clearly in excess of normal expectancy
 Common Vehicle Epidemics : A common vehicle epidemic
occurs due to presence of infectious or a chemical noxious
agent in a common vehicle which serves as the channel of
transmission,

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(A) Common Source (Vehicle), Single (Point) Exposure:
 The infective material remains present in the vehicle for a brief
period of time; during this period all those who come in contact with
the vehicle become exposed to the infection.eg outbreaks of food
poisoning and of cholera.

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 (B) Common Source, Continued exposure : Such an epidemic
would occur when an infectious agent persists in the common
vehicle for some amount of time.eg contamination of surface /
ground or piped water supplies with human excreta, as in
infectious hepatitis or cholera, or food borne typhoid fever
outbreaks due to carriers or contaminated tinned foods

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 (C) Common Source, interrupted exposure :
There is a common source, but the source introduces the infection into
the vehicle only interruptedly.
For example, out of the 4 nurses looking after a urological ward, 1
maybe carrier of Pseudomonas aeroginosa.
Now, as and when this particular sister is on duty, she would introduce
the infection through the catheters
Such a curve will show an increase in frequency but the curve will be
almost flat with occasional irregular waves coinciding with the
periodic introductions of infection.

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 2. Propagated Source : the source itself propagates, i.e.
multiplies. For example, in a group of schoolchildren, the first
(index) case of diphtheria may pass on droplet infection to 3
others; these 3 may then pass on to 9 others and thus the source
multiplies. Eg Diphtheria, Mumps, Measles, Malaria, JE and
Dengue epidemics, Influenza and in STD and HIV epidemics.

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 Seasonal fluctuations : Malaria and JE are commoner during
immediate post monsoon season due to increased breeding of vectors.
Similarly, asthma shows highest incidence during spring and autumn
suggesting specific environmental factors in its causation,eg Maxcy s
findings
 Cyclical Changes : These are periodic peaks in disease frequencies
occurring every 3 - 5 years. The common example is of measles in
which epidemics tend to occur in cycles of 2 – 3 years, possibly due to
accumulation of enough susceptibles.

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 Secular trends : These are time trends occurring over a period
of decades. Secular trends during this century have been
noticed for cancers of various sites - a declining trend in death
rate due to cancers of stomach and uterus and a rising trend for
cancers of lung and pancreas, while there is no change in breast
cancer mortality rate.

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DISTRIBUTION ACCORDING TO PLACE
 Studies of the geography of disease (or geographical
pathology) is one of the important dimensions of descriptive
epidemiology.
These variations may be classified as :
 a. International variations
 b. National variations
 c. Rural - urban differences
 d. Local distributions

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INTERNATIONAL VARIATIONS
Japan has very low CHD mortality rates but high rates for
cerebro - vascular accidents, Hypertension and gastric CA; UK
has high lung CA rates while USA has high CHD rates.

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National, Regional or International differences in the
availability of medical services, diagnostic practices
of physicians, quality of available diagnostics,
differences in disease classification procedures,
completeness of reporting of death & disease and
completeness of census may all introduce artifactual
differences,can be assessed by migrant studies

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 NATIONAL VARIATIONS
Variations in the disease occurs within countries or national
boundaries.
Death statistics and notification of infectious disease are
major sources of data for comparison within countries.

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 RURAL – URBAN VARIATION
These variations may be due to differences in population
density, social class,deficiences in medical care,levels of
sanitation,education, environmental factors.
Chronic bronchitis, accidents, lung cancer, cardio-vascular
diseases, mental illness and drug dependence are more frequent
in urban than in rural areas.
On the hand, skin and zoonotic diseases and soil-transmitted
helminths may be more frequent in rural areas than in urban.

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LOCAL VARIATION
 This is the differences in disease occurrence according to
place within small, defined localities.
 For examining such differences, the epidemiologist makes a
“spot map” which is a detailed layout map of that area or
locality
 1. The inhabitants of that place, by virtue of their
genetic factors or socio economic factors, are different
from those at other places.

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 2. Some etiologic factors, characteristic in the place are present. If this
is the reason, then :
(i) High rates of disease will be observed in all ethnic groups in that
area.
(ii) High rates are not observed in persons of similar ethnic groups
living in other areas.
(iii) Healthy persons entering that area become ill with a frequency
similar to the indigenous inhabitants.
(iv) Inhabitants who have left that area do not show high rates.
(v) Some evidence of the disease may also be found in animals in the
same area.

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MIGRANT STUDIES
 Let us say a group of people ‘M’, have immigrated from the original
country ‘X’ to a new country ‘Y’.
 Let us say, the mortality (or morbidity) rates due to a given disease of
interest ‘A’ are as follows :Rates in original country = ‘X’; Rates in new
country = ‘Y’;Rates among the migrants = ‘M’.
 Now, if the differences in a disease are due to environmental factors, then
‘M’ would approximate ‘Y’ but ‘M’ would tend to be different than ‘X’.
 On the other hand, if the disease is determined by genetic factors, ‘X’ and
‘M’ will be equal, and ‘X’ and ‘M’ will be different from ‘Y’.

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DISTRIBUTION ACCORDING TO PERSON
AGE
 Age is a person related variable that is almost universally
studied.
 “Bimodality”,i.e. two peaks in the age distribution of
morbidity or death due to a particular disease
 Certain diseases are more frequent in certain age groups
than in others, e.g., measles in childhood, cancer in middle
age and atherosclerosis in old age.

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Sex
 The sex related differences may be due to hormonal or other
biological differences or due to differences in attitude towards
life.
 Sex linked genetic inheritance, and (b) cultural and behavioural
ETHNIC GROUP
 Ethnic group is defined as a group of persons who have a
greater degree of homogeneity than the population at large in
respect of biologic inheritance and present day customs.
 ● Race.
 ● Nativity
 ● Religion
 ● Local reproductive and social units

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SOCIAL STATUS
 Social status may be an independent risk factor for the disease
or it may be indirectly associated with the disease in question
OCCUPATION
 The stress of occupation and exposure to various physical,
chemical and biological disease agents therein, may be
associated with high occurrence of such diseases.. In case of
studies on children occupation, education and income of the
father and mother .

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EDUCATION
 Education leads to an improved level of knowledge and hence
is likely to be associated with reduced risk of disease.
MARITAL STATUS
 In general, mortality rates are lowest among married, followed
by single, widowed, and divorced, in that order, irrespective of
sex.
 Marital status is also a risk factor for some disease and
condition.eg cancer cervix is rare in nuns.

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TWIN STUDIES
 Concordance of a disease in monozygotic twins as compared to
dizygotic is a strong indication of genetic background and
discordance point towards environmental factors
BEHAVIOUR
 The behaviourial factors which have attracted the greatest
attention smoking. sedentary life, over-eating and drug
STRESS
 Stress has been shown to affect a variety of variables related to
patients response,

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TYPES OF DESCRIPTIVE STUDY
CASE REPORT
 The case report is the least publishable unit in the medical
literature.
 Observant clinician reports an unusual disease or association,
which prompts further investigations with more rigorous study
designs .
 For example, a clinician reported benign hepatocellular
adenomas, a rare tumour, in women who had taken oral
contraceptives.

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Case-series report
 A case-series aggregates individual cases in one report.
 For example, a cluster of homosexual men in Los Angeles
with a similar clinical syndrome alerted the medical
community to the AIDS epidemic in North America.
 A convenient feature of case-series reports is that they can
constitute the case group for a case-control study, which can
then explore hunches about causes of disease.

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CROSS SECTIONAL STUDIES
 Snapshot of the population at a certain point in time.
 The design is also called a prevalence study.
 We define a population and determine the presence or
absence of exposure and the presence or absence of
disease for each individual. Each subject then can be
categorized into one of four possible subgroups.

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Advantages of Cross-sectional Studies
 Used to study conditions that are relatively frequent with long
duration(chronic conditions)
 Good for generating hypotheses; for the cause of disease
 Can estimate overall and specific disease prevalence rates
 Can estimate exposure proportions in the population.
 Relatively easy, quick, inexpensive, and ethically safe

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Disadvantages of Cross-sectional Studies
 Impractical for rare diseases
 Not a useful study type to establish causal relationships
 Does not allow to determine which came first
 Not suitable for studying highly fatal diseases or a disease with
short duration of expression

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LONGUITUDINAL STUDY
 These studies are undertaken with the following general
objectives :
 i) To see the incidence of a disease
 ii) To describe the ‘natural history of a disease’
 iii) To describe a health related natural phenomena
 iv) To study the ‘trend’ of a disease and trend of health related
phenomena

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ECOLOGICAL STUDIES
Correlational studies look for associations between
exposures and outcomes in populations rather than in
individuals.
Limitations—ie, the inability to link exposure to outcome
in individuals and to control for confounding (a mixing or
blurring of effects).

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 Ecological fallacy can be defined as the state when an
association which has been seen among variables, on an
aggregate level may not necessarily represent an association at
the individual.
 Ecological studies can be of two types, viz., cross – sectional
and longitudinal

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MERITS AND DEMERITS
 The data are already available and thus inexpensive and
efficient to use
 Only few ethical difficulties exist.
 Forwards tentative guesses (hypothesis) about the possible
causal role of certain factors in the outcome of the interest,
 Study also intends to describe the “distribution” of such
means, proportions or natural progress of health – related
phenomena

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 Descriptive studies have important limitations.
 Temporal associations between putative causes and effects
might be unclear.
 A dangerous pitfall is that the investigators might draw causal
inferences when none is possible
 The study is done only on one group of subjects; there is no
comparison group
 Incidence and risk cannot be calculated

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USES
 Trend analysis
 Provides background data for planning ,organizing and evaluating
preventive and curative services
 Provide data regarding magnitude of the disease load and types of disease
problems in the community in terms of morbidity and mortality ,rates ratios
 Provide clues to disease aetiology, and help in formulation of an
aetiological hypothesis
 They contribute to research by describing variation in disease occurrence by
time, place and person

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CONCLUSION
 Descriptive studies are often the first, tentative approach to a new
event or condition.
 These studies generally emphasise features of a new disease or
assess the health status of communities.
 Health administrators use descriptive studies to monitor trends
and plan for resources.
 By contrast, epidemiologists and clinicians generally use
descriptive reports to search for clues of cause of disease—i.e.,
generation of hypotheses.

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THANK YOU