Epidemiology uses two approaches: asking questions and making comparisons. Descriptive epidemiology describes disease occurrence and distribution by time, place and person. It involves defining the population and disease, describing disease characteristics, measuring disease occurrence, and comparing groups. Analytical epidemiology tests hypotheses, using case-control and cohort studies to compare exposure rates between cases/controls and exposed/unexposed groups. Experimental studies manipulate interventions through randomized controlled trials, field trials and community trials.

1. EPIDEMIOLOGY

2. TOPICS: -
 Epidemiological Approaches
 Methods of Epidemiology

3. Epidemiological Approaches
The epidemiological approach to problem of health is based on
two measure foundations:
Asking
questions
Making
Comparisons

4. Asking Questions: -
 The epidemiologists ask a variety of questions and
make observations related to nature and extent of
problem, geographical distribution, time trend and
personal characteristics of the person who gets the
disease.

5. Contd…
Related to Health Events:
What is the event?
What is its magnitude?
Where did it happen?
When did it happen?
Who are affected?
Why did it happen?

6. Contd…
Related to Health Action:
What can be done to reduce this problem and its
consequences?
How can it be prevented in the future?
What action should be taken by the community?
What resources are required?
What difficulties may raise, and how might be they
overcome?

7. Making Comparisons: -
 The basic approach in epidemiology is to make
comparisons and draw inferences.
 Epidemiological approach is not only for an
individual, but also for whole population .

8. Contd…
• It is expressed in term of
a)Rate = Data of illness
Data of whole population at risk
b)Prevalence Rate =Number of people with the Disease at a
specified time. X 10n
Number of people in the population at
risk at the specified time

9. Contd…
The prevalence rate is often expressed as cases per 1000 or per
100,000 populations:
• Point prevalence rate: Data collected at one point of time.
• Period prevalence rate: Data collected during a specified
period.

10. Contd…
c)Incidence Rate = Number of people who get a disease (new
cases) in a specified period.
Sum of the length of time during which
each person in population is at risk.

11. EPIDEMIOLOGICAL METHODS

12. Epidemiological methods are classified in to two:
Observational
Descriptive
Analytical
Experimental
Randomized
Community and
field Trials

13. Descriptive Epidemiology: -
 Descriptive epidemiology is concerned with occurrence, size,
distribution and progression of health and disease in population.
 These studies are usually the first phase of an epidemiological
investigation.
 It gives information about who all are affected by a particular
disease.

14.  Following characteristics are examined in descriptive study:
 Time- Year, Season, Month, Week, Day, Hour of onset,
Duration.
 Place- Climate zone, country, urban or rural region, local
community, towns, cities, institutions.
 Person- Age, Sex, Marital status, Occupation, Social status,
education, height, weight, family size.

15. STEPS IN DESCRIPTIVE STUDIES
STEP 1
Define
Populati
on
STEP 2
Define
disease
STEP 3
Describe
disease
by time,
place
and
person
STEP 4
Measure
-ment of
Disease
STEP
5
Comp
aring
the
known
indice
s
STEP 6
Formul
ation of
Hypoth
esis

16. Defining the population to be studied: -
 The first step is to define the population base not only in terms
of the total number but also in composition in terms of age, sex,
occupation and cultural characteristics.
• The defined population needs to be large enough so that age,
sex and other specified rates are meaningful.

17. Contd….
• The community should be stable, without migration into and
out of area.
• Also the population should not be overtly different from other
communities in the region.
• Finally, a health facility should be close enough to provide
easy access for patients requiring medical facilities.

18. Defining the disease under study: -
• Once the population to be studied is defined, one must then
define the disease being investigated.
• The clinician may not need a precise definition of disease for
immediate patient care. If the diagnosis is wrong, he can
revise it subsequently.

19. Contd….
• But the epidemiologist, who main concern is to obtain an
accurate estimate of the disease, needs precise and valid
definition to enable it.
• So the epidemiologist looks for operational definition, i.e. the
definition which can be identified and measured in defined
population with a degree of accuracy.

20. Describing the Disease by time, person, place: -
• The primary objective of descriptive epidemiology is to describe
the occurrence and distribution of disease by time, place and
person.
 The time Distribution: -
• The pattern of disease may be described by time of its
occurrence.

21. Contd….
• The epidemiologists have identified three kinds of time trends or
fluctuations in disease occurrence:
i. Short term fluctuations.
ii. Periodic fluctuations
iii. Long term or secular trends.

22. Contd….
i. Short term fluctuations: -
• The best short term fluctuations are occurrence of an
epidemic.
• An epidemic is the occurrence in a community of an illness or
other health related events clearly in excess of normal
expectancy.

23. Contd….
ii. Periodic fluctuations: -
A. Seasonal trend: For example, measles is at its height in
early spring. Upper respiratory infections rise in winter months. GI
infections are prominent in summer because of warm weather and
rapid multiplication of flies.

24. Contd….
B. Cyclic trend: Some diseases occur in cyclic order over a short
period of time which may be days, weeks, months or years. For
example, measles in the pre vaccination era appeared in cycles
with major peaks in every 2 to 3 years and rubella every 6 to 9
years.

25. Contd….
iii. Long term or Secular Trends: -
The term “secular trend” implies changes in occurrence of
disease over a long period of time, generally several years or
decades.

26. Contd….
 Place Distribution: -
• Studies of geography of disease are one of the important
dimensions of descriptive epidemiology.
• By studying the distribution of disease in different population,
we gain perspective on the fascinating differences in disease
pattern not only between countries but also within countries.

27. Contd….
• The variation can be:
a) International variation.
b) National variation.
c) Rural- Urban differences.
d) Local distribution.

28. Contd….
 Person’s Distribution: -
• In descriptive studies the disease is further characterized by
defining the person who develops the disease by age, sex,
occupation, marital status, habits, social class and other host
factors.

29. Contd….
Measurement of Disease: -
• It is mandatory to have a clear picture of the
amount of disease in the population.
• This information should be available in terms of mortality,
morbidity, disability and so on.

30. Contd….
Comparing with the Known Indices: -
• The essence of epidemiology is to make comparisons and ask
questions.
• By making comparisons between different populations and
subgroups of same population, it is possible to arrive at clues to
disease etiology.

31. Formulation of Hypothesis: -
• An epidemiological hypothesis should specify the following:
a) The population: The characteristics of persons to whom the
hypothesis implies.
b) The specific cause being considered.
c) The expected outcome of disease.
d) The dose response relationship.
e) The time response relationship.

32. Analytical Epidemiology: -
 Analytical studies are second major type of epidemiological
studies.
 The subject of interest in analytical studies is an individual within
the population.
 The object is not to formulate but to test the hypothesis.

33.  It comprises two distinct types of observational studies:
• Case- control studies.
• Cohort studies.
Case- control studies: -
• Case- control study often called “retrospective studies” is a
common first approach to study of test causal hypothesis.

34. • It has three distinct features:
 Both exposure and outcome have occurred before the start
of the study.
 The study proceeds backward from effect to cause.
 It uses a control or comparisons group to support or refute
an inference

35. Contd…..
• Case- control study involves two populations: case and
control.
• Case control studies are basically comparison studies.

36. Contd…..
• The four basic factors in conducting a case- control study
Selection of case
and control
Matching
Measurement of
Exposure
Analysis and
Interpretation

37. Contd…..
Selection of cases and control: -
• The first step is to identify a suitable group of cases and group
of controls.
Definition of Cases:
• According to the diagnostic criteria: The diagnostic criteria of the
disease and the stage of the disease and the stage of disease

38. Contd…..
to be included in the study must be specified before the study is
undertaken.
• According to the eligibility criteria: A criterion customarily
employed is the requirement that only new cases within the
specified period of time are eligible.

39. Contd…..
Sources of cases:
• The cases may be drawn from hospitals and general population.
Selection of controls:
• The control must be free from the disease under the study.
They must be similar to the cases as possible except for the
absence of disease.

40. Contd…..
Sources of controls:
• The possible sources from which controls can be selected are
hospitals, neighbor's, relatives and general population.
Matching: -
• Matching is defined as the process by which we select control in
such a way that they are similar to cases in regards to certain

41. Contd…..
pertinent selected variables, which are known to influence the
outcome of the disease and which, if not adequately matched,
could distort or confound the results.
• There are mainly two type of matching procedures:
 Group matching.
 Individual matching.

42. Contd…..
Measurement of Exposure: -
• Definition and criteria about exposure are just as important
as those used to define cases and controls.
• Information about exposure can be obtained by interviews,
by questionnaire or by studying the past record.

43. Contd…..
Analysis: -
• The final step is an analysis to find out:
 Exposure the rates among cases and controls to the
suspected factors.
 Estimation of disease risk associated with exposure.

44. Contd…..
Advantages of retrospective studies:
• Inexpensive and Easy
• Require fewer subjects
• Useful for rate of disease or diseases with long latency
periods.
• Can check for more than one risk factor.
• Useful for drug induced illness.

45. Contd…..
Disadvantages of retrospective studies:
• Information not easily available.
• Difficult to validate.
• Quality of survey.
• Bias in selection.
• Recall may be flawed.

46. Contd…..
Cohort studies: -
• Cohort study is another type of analytical study which is
usually undertaken to obtain additional evidence to refuse or
support the existence of an association between suspected
cause and disease.
• It is also known as prospective study or incidence study or
forward looking study.

47. The steps or elements of cohort study include
Analysis
Follow up
Selection of
comparison group
Obtaining data on
exposure
Selection of study subject

48. Contd…..
Selection of study subjects:
• General population
• Specific groups like nurses or teacher.
Obtaining Data on exposure:
• Through interviews, mailed questionnaires, review of records,
medical examination and environmental survey.

49. Selection of comparison Groups:
• Internal comparisons and External comparisons
• Comparison with general population rates.
Follow- up: Periodic medical investigation, hospital records,
telephone calls, mailed questionnaires and home visits.
Analysis: Incidence rate of outcome among exposed and non-
exposed.

50. Advantages of cohort study:
• It can determine incidence rates and attributable risk directly.
• Exposure can be determined with less potential for bias than
if outcome were already known.
• It can be efficient for studying rare exposures.
Disadvantages:
• Inefficient and Expensive
• Bias due to attrition or loss to follow- up.

51. Experimental studies:
• Experimental studies involve some action, intervention or
manipulation in the experimental groups while making no
changes in the control groups and comparing the outcome of
the experiment in both groups.

52. • Types: -
a) Randomized Controlled Trials.
b) Field trials.
c) Community Trials.

53. Randomized Controlled Trials:
• It is a form of human experimentation in which the subjects,
usually patients, are randomly allocated to receive either a
standard accepted therapeutic or preventive regimen and
analysis done.

54. • The basic steps in conducting RCT are:
Drawing up a protocol.
Selection Reference and experimental population.
Randomization
Manipulation or intervention.
Follow- up.
Assessment of outcome.

55.  Field Trials: Use people as unit of study.
 Community Trials: Use community as unit of study.

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