BREIF overveiw of Descriptive studies ; their epidemiological uses

1. I
TIME
PLACE
PERSON
DESCRIPTIVE
STUDIES
Presented by ,
Dr K Himabindu Reddy
JR1 ,
Dept of Community medicine
DMIMS , Wardha
Guided by ,
Dr Abhishek Joshi
Associate Professor,
Dept of Community medicine
DMIMS , Wardha

2. Epidemiological Studies
Observational Studies
Descriptive Studies
Case reports
Cross-sectional
Descriptive Studies
Longitunidal
Descriptive studies
Ecological Studies
Analytical
Studies
Cross-sectional Studies
Case-Control studies
Cohort Studies
Experimental /
Interventional Studies
Randomized Control
Trials
Field Trials
Community Trials

4. Types of Descriptive studies
 Case Reports and Case Series
1. This type of study is based on reports of a single, or else a series of cases of specific treated or
untreated condition without any specific comparison (control) group.
2. Apart from describing symptoms in series of patients , we may also work out “proportions” (e.g.
percentage of cases that belong to a particular age group, sex, ethnic group etc.).
• However, we must remember that these proportions do not indicate risk since the denominator is still the number of cases
and not the population at risk
 Cross Sectional Descriptive Studies
Cross sectional descriptive studies are done on a sample of the total population and may be
community based or hospital based. They are mainly directed to work out the :
1. “Prevalence” of a factor of interest.
2. “Mean” of a factor of interest.
3. Description of a “Pattern” .
4. As a surrogate for longitudinal descriptive studies

5.  Longitudinal Descriptive studies
1. More scientific than Cross sectional ones but at the same time more costly and time
consuming.
2. In contrast to a cross sectional descriptive study, a longitudinal descriptive study follows
up a single group of subjects over a defined period of time.
3. General Objectives of Longitudinal Descriptive study:
a) To see the incidence of a disease.
b) To describe the ‘natural history of a disease’.
c) To describe a health related natural phenomena.
d) To study the ‘trend’ of a disease.
e) To study the ‘trend of a health - related phenomena.
 A Cross - sectional study gives us the “prevalence” while A Longitudinal study gives us
the “incidence”.
Types of Descriptive studies

6. 1)Defining population to be studied.
Definingdisease under study.
Describing disease by *Time *Place *Person.
Measurement of the disease.
Comparing with known indices.
Formulation of an aetiological hypothesis
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7. Defining Population to be studied
 Descriptive studies are investigations of populations, not individuals. 1st step hence , is to define the ‘ Population
base’.
he concept of 'defined population' (or population at risk) is crucial in epidemiological studies. It provides the denominator for calculating rates which are essential to measure the frequency of disease and study
its distribution and determinants.
 The "defined population" can be the whole population in an area, or a representative sample taken from it.
 The defined population needs to be large enough so that age, sex and other specific rates are meaningful.
 The community chosen should be stable, without migration into or out of the area.
 Community participation is an essential component.
 The population chosen , should not be overtly different from other communities in the region.
A health facility should be close enough to provide relatively easy access for patients requiring medical services.

8. Defining the disease under study
 Once population to be studied is specified, define disease to be studied.
 The needs of clinician & epidemiologist vary while defining the disease.
 The epidemiologist needs an "operational definition", by which the condition can be identified and measured
in the defined population with good degree of accuracy.
 The diagnostic methods for use in epidemiological studies must be acceptable to the population to be studied,
and applicable to their use in large populations.
 With regard to certain diseases (e.g., neurological diseases) which often do not have pathognomonic signs and
symptoms, disease definition is a crucial concern for the epidemiologist. In such cases, the epidemiologist
frames his own definition keeping the objectives of his study in view and aiming at the same time a degree of
accuracy sufficient for his purpose.
 Once established, the case definition must be adhered to throughout the study.

9. Describing disease by Time, Place, Person
Person related Variables
• Age
• Sex
• Ethnic group
• Social class
• Occupation
• Education
• Marital Status
• Family variables
• Twin studies
• Other variables

10. Describing disease by Time, Place, Person
According to
Calendar time
Short-term (Epidemic)
fluctuations
Common Source
Propagated
Source
Long-term (Secular)
trends
Changes in disease over a
period of decades
According to natural
time / non-calendar
Seasonal Fluctuation
Cyclic Fluctuation

11. Graphical representation of Epidemic curves
Fig: Common Vehicle , Point source epidemic curve

12. Graphical representation of Epidemic curves
Fig: Common Vehicle , Point source epidemic curve

13. Graphical representation of Epidemic curves
Propagated
epidemic Curve

14. Graphical representation of Epidemic curves

15. Describing disease by Time, Place, Person
 Many diseases have typical spatial relationships. Geographical pathology is an important
dimension of descriptive epidemiology.
 Differences in the distribution of a disease according to place may be made according to Political
boundaries or according to Natural boundaries.
 Geographic patterns provide an important source of clues about the causes of the disease. The
range of geographic studies include those concerned with local variations.
a) International- National variations
b) Rural –urban variations
c) Local distributions
 When making such international comparisons in respect of a disease, one must initially assess
whether these observed differences are artifactual.
 We must make sure that the differences are not artifactual, but real, [ either due to the play of
environmental factors or else genetic factors], a good method of dissecting this out are “Migrant
Studies”.

16. Describing disease by Time, Place, Person
 Migrant studies can be carried out in two ways :
a) Comparison of disease and death rates for migrants with those of their kin who have stayed
at home.
• If the disease and death rates in migrants are similar to country of adoption over a period of
time, the likely explanation would be change in the environment.
b) Comparison of migrants with local population of the host country.
• We get information on genetically different groups living in a similar environment. If the
migration rates of disease and death are similar to the country of origin, the likely explanation
would be the genetic factors.
 Regional Variations within countries : Regional differences help in developing hypothesis
about role of possible environmental agents in the etiology of the disease. Eg : Goitre
 . Rural - Urban differences point out towards possible environmental factors; e.g. IHD, STDs,
Hypertension etc. are more common in the urban areas while oro - faecal infections are more
common in rural areas.

17. Describing disease by Time, Place, Person
 Local distributions : Most often, the epidemiologists have to study local distributions,
i.e. 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, showing the accommodation, water sources and supply lines, nightsoil disposal systems, vector
breeding areas, eating establishments and various other environmental factors of relevance.
 On the same map, the epidemiologist plots the cases of the disease according to their frequency, looking for the
places where there is a high frequency of cases and then trying to relate them with the possible environmental
factors.
 Proper knowledge of use of ‘spot map’ is necessary while investigating most of the epidemic outbreaks

18. Describing disease by Time, Place, Person
Spot map of Asiatic cholera in London

19. Describing disease by Time, Place, Person
Methods of Displaying & Analysing Place related disease:
1. Spot Mapping
2. Map-on-map :
In this technique we combine two maps to bring disease frequencies, plotted as coloured dots, into visual
approximation with other variables like roads, rivers, indices of poverty etc. This technique may also be used
for studying “movement” of a disease in both time and place.

20. 1)Defining population to be studied.
Definingdisease under study.
Describing disease by *Time *Place *Person.
Measurement of the disease.
Comparing with known indices.
Formulation of an aetiological hypothesis
1
2
3
4
5
6

21. Measurement of the disease
 Its necessary to have a clear picture of the disease load in the population.
 Information about the disease load should be available in terms of morbidity ,mortality &
disability.
 Measurement of mortality is straightforward.
 Morbidity has 2 aspects : incidence & prevalence. Incidence can be obtained from ‘longitudinal
studies’ & prevalence from ‘cross-sectional studies’.
 Descriptive epidemiology thus uses a cross-sectional or longitudinal design to estimate
magnitude of health and disease problems in human populations.

22. Measurement of the disease
 CROSS-SECTIONAL STUDY : [a.k.a Prevalance study]
 Simplest form of observational study.
 Single examination of a cross-section of the population at 1 point in time: results of which are
projected on whole population.
 More useful for chronic , than short lived study.
 A cross-sectional study provides information about disease prevalence, but not enough
information about the natural history of disease or about the rate of occurrence of new cases
(incidence).
 LONGITUDINAL STUDY :[a.k.a Incidence study ]

23. Comparing with known indices
 The essence of epidemiology is to make comparisons and ask questions.
 Comparisons between different populations, and subgroups of the same population, give clues to disease
aetiology.
 We can also identify or define groups which are at increased risk for certain diseases.

24. Formulation of a Hypothesis
 A hypothesis is a supposition, arrived at from observation or reflection.
 It can be accepted or rejected, using the techniques of analytical epidemiology.
 An epidemiological hypothesis should specify the following:
1. The population - the characteristics of the persons to whom the hypothesis applies
2. The specific cause being considered .
3. The expected outcome - the disease .
4. The dose-response relationship - the amount of the cause needed to lead to a stated
incidence of the effect
5. The time-response relationship - the time period that will elapse between exposure to the
cause and observation of the effect.
 The success or failure of a research project frequently depends upon the soundness of the
hypothesis

25. 1) Provide data about the types of disease related problems in the community in terms of
morbidity and mortality rates.
2) Help in the formulation of an etiological hypothesis.
i.e, the existence of a possible causal association between a factor and a disease is usually
recognized in descriptive studies. Thus, if the disease is observed to be more frequent in a
particular group than in others, hypotheses are formulated to explain the increased frequency
3) Provide background data for planning, organizing and evaluating preventive and curative
services.
4) Contribute to research by describing variations in disease occurrence by time , place and
person

26. 1) Since there is no information of the population at risk , nor a comparison group, neither can risk be
calculated nor can a hypothesis be tested
2) Ecological fallacy

27. Difference between Descriptive & Analytical study
SL.No DESCRIPTIVE STUDY ANALYTICAL STUDY
1. Only one group is studied At least 2 groups are studied to draw comparisions.
2. Before the study , there is no explicit
hyposthesis regarding cause-effect
relationship.
At the start of the study ,there is a definite
hypothesis , regarding an exposure possibly
causing an outcome
3. The study ends with development of
possible hyposthesis regarding cause and
effect relationship .
At the end of study , we confirm or reject the
hypothesis with which the study was started.

28. Thank You
References
 Park, K., 2018. Parks Textbook Of Preventive In Social Medicine. 24th
ed. P47 - 51 Jabalpur: Banarsidas Bhanot Publishers.
 Rajvir Balwar , 2009. Textbook of Public Health and Community
Medicine. 1st ed. P. 23 -25 Pune: Armed Forces Medical College and
WHO (collaboration).