The document discusses key concepts in epidemiology including descriptive studies, analytical studies, and interventional studies. Descriptive studies involve systematically collecting and presenting data to describe a health situation, such as the occurrence of a disease over time, place, and person. Analytical studies attempt to establish causes or risk factors by comparing groups with and without a problem. Common analytical study designs include case-control and cohort studies. Interventional studies involve manipulating a situation and measuring the effects, with experimental studies using randomization and control groups.

2. The 6 D’s of Health
Outcomes
□ Death
□ Disease
□ Discomfort
□ Disability
□ Dissatisfaction
□ Destitution
□ A bad outcome if Untimely
□ A set of Symptoms, Signs and Lab results
□ Symptoms such as pain, nausea and itching
□ Impaired ability to go about usual activities
□ Emotional reaction to disease and its care
□ Financial Cost of Illness

3. Clinical
Questions
● Abnormality
● Frequency
● Diagnosis
● Cause
● Risk
● Prognosis
● Treatment
● Prevention
● Cost
Is the patient sick or well
How often does disease occur
How accurate are the diagnostic tests
What conditions lead to disease
What factors are associated with the disease
What are the consequences of disease
How does treatment change the course
Does an intervention prevent disease
Does an early detection improve the course
How much will care cost

4. Non Interventional Studies
(Observational)
Interventional Studies
(Experimental)

5. Epidemiological
Studies
● Descriptive Studies
● Describes Disease occurrence in a population
● Incidence
● Prevalence
● Survivial
● Analytic Studies
● To determine etiology of disease
● Cohort
● Case Control
● Cross Sectional

6. EPIDEMIOLOICAL METHODS
OBSERVATIONAL STUDIES
(NON INTERVENTIONAL)
EXPERIMENTAL STUDIES
(INTERVENTIONAL)
Descriptive
studies
Analytical
Studies
Cross Sectional Case Control
Cohort
Randomized
Controlled Trials
Field trials
Community Trials
Longitudnal

7. Descriptive
•Case report
•Case series
•Survey
Analytic
Observational
•Cross sectional
•Case-control
•Cohort studies
Experimental
•Randomized
controlled trials
Strength of evidence for causality between a risk factor and outcome

8. Epidemiological sequence
□ Observation – data collection
□ Counting cases and events
□ Relating cases and events to population at risk
□ Making comparison
□ Developing hypothesis
□ Testing of hypothesis by analytical studies
□ Making scientific inferences
□ Conducting experimental studies
□ Interventions/Evaluation

10. 1. DESCRIPTIVE STUDY
FIRST THING’S FIRST…..!!!!

11. Definition
It involves the systematic collection and presentation
of data to give a clear picture of a particular situation.
Descriptive can be carried out on small or large scales
in community

12. Concerns
Descriptive study is concerned with the following
questions:
a. When is the disease occurring (Time)
b. Where it is occurring (Place)
c. Who is getting the disease (Person)
T.P.P

13. Uses
□ Provides data regarding
The magnitude of the disease load
The types of disease problem in the community in term
of morbidity and mortality rate and ratio
□ Provides “clues” to the disease etiology
□ Helps in
Formulation of etiological hypothesis
Helps in planning, implementation and evaluation of
health services/programmes

14. Procedures
□ Defining the population to be studied
□ Defining the disease under study
□ Describing the disease by
Time
Place
Person
□ Measurement of the disease
Cross Sectional Study
Logitudinal Study
□ Comparing with known indices
□ Formulation of an etiological hypothesis

15. Measurement of Disease
a. Cross Sectional Study
b. Longitudinal Study

16. Cross sectional studies
Based on a single examination of a
cross section of population at one
point in time, results of which can be
projected on the whole population
provided the sampling has been done
correctly.

17. Uses
a.More useful in chronic diseases
b. To find more about disease rather than its
etiology

18. Characteristics
□ Physical characteristics of people, material and
environment
□ Socio-economic characteristics e.g., age, education ,
marital status, number of children and income
□ Behavior of people like knowledge, attitude and
beliefs (KAP)
□ Events that occur in population

19. Advantages of
CSS
□ May study several outcomes
□ Control over selection of subjects
□ Control over measurements
□ Relatively short duration
□ First step for cohort study
□ Yields prevalence

20. Disadvantages of
CSS
● Does not establish cause/effect ratio
● Potential bias in measuring exposure
● Potential survival bias
● Not feasible for rare disease
● Does not yield incidence

21. Longitudinal
Study
●Based on multiple observations in the same
population over a prolong period of time.

22. Uses of Longitudinal
Study
● Natural History of Disease
● Identifying Risk factors
● Finding out incidence rate

23. Cross Sectional Vs Longitudinal

25. COMPARATIVE or ANALYTICAL
STUDY
An ANALYTICAL
establish causes
STUDY
or risk
certain problems. This is done
attempts to
factors for
by
comparing two or more groups, some of
which have or develop the problem and
some of which have not.

26. Analytic
Epidemiology
□ Second major type of epidemiological studies
□ Subject of interest is individual within the
population
□ The objective is to test hypothesis
□ The study determines whether or not a statistical
association exists between a disease and suspected
factor
□ Strength of association, if it exists

27. CASE CONTROL STUDY
In a CASE-CONTROL STUDY, the
investigator compares one group among
whom a problem is (e.g., malnutrition)
with another group, called a control or
comparison group, where the problem
is absent to find out what factors have
contributed to the problem.

28. CASE CONTROL STUDY
● Often called retrospective study
● First approach to test causal hypothesis

29. Properties
● Both exposed and outcome (Disease) have
occurred before the start of study
● The study proceeds backward from effect to cause
● It uses a control or comparison group to support or
refute an inference

30. Method
□ Selection of cases and Controls
□ Matching
□ Measurement of Exposure
□ Analysis and Interpretation
Exposure Rates
Estimation of Risk (Relative Risk & Odds Ratio)

31. A
C D
B
Cases
Exposure -
Exposure +
Controls
A+C B+D
2 x 2 Contingency Table for Cases and Controls
Total Cases
Exposure Among Cases
A/(A+C)
Exposure Among Controls
B/(B+D)

32. Advantages of case control study
□ Relatively easy to carry out
□ Rapid and inexpensive
□ Particularly suitable to investigate rare diseases
□ No risk to subject
□ Reveals the study of several different etiological
factors
□ Risk factors can be identified
□ No follow up in the future
□ Minimum ethical problems

33. Disadvantages of case control study
● Problems of bias
●Selection of appropriate case control group may
be difficult
●Cannot measure incidence, only relative risk is
measured

34. Randomized controlled trials
●Investigator controls the predictor variable
(intervention or treatment)
●Major advantage over observational studies is ability
to demonstrate causality
● Randomization controls unmeasured confounding
● Only for mature research questions

35. Population
Sample
Treatment Dx No
Dx
C
Co
on
nt
tr
ro
ol
l Dx No
Dx
Placebo
Randomization

36. Steps in a randomized controlled trial
1. Select participants
●
●
●
high-risk for outcome (high incidence)
Likely to benefit and not be harmed
Likely to adhere
1. Measure baseline variables
2. Randomize
●
●
Eliminates baseline confounding
Types (simple, stratified, block)

37. Analysis of randomized controlled
trial
● Analyzed like cohort study with RR
● Intention to treat analysis
● Most conservative interpretation
● Include all persons assigned to intervention group
(including those who did not get treatment or dropped
out)
● Subgroup analysis
● Groups identified pre-randomization

38. Steps in a randomized controlled trial
4. Blinding the intervention
●
●
As important as randomization
Eliminates
●
●
●
co intervention
biased outcome ascertainment
biased measurement of outcome
5. Follow subjects
●
●
Adherence to protocol
Lost to follow up
6. Measure outcome
●
●
Clinically important measures
Adverse events

39. What is Blinding?
□ Single blind - participants are not aware of
treatment group
□ Double blind - both participants and
investigators unaware
□ Triple blind - various meanings
persons who perform tests
outcome adjudicators
safety monitoring group

40. Why blind?: Co
interventions
□ Unintended effective interventions
■ participants use other therapy or change behavior
■ study staff, medical providers, family or friends
treat participants differently
□ Nondifferential - decreases power
□ Differential - causes bias

41. Why blind?: Biased Outcome
Ascertainment or adjudication
• participants may report symptoms or outcomes
differently
• physicians or investigators may elicit symptoms or
outcomes differently
• Study staff or adjudicators may classify similar events
differently in treatment groups
□If group assignment is known
□ Problematic with “soft” outcomes
investigator judgement
participant reported symptoms, scales

42. High Quality Randomized
Trials
● Tamper-proof randomization
●Blinding of participants, study staff, lab
staff, outcome ascertainment and
adjudication
●Adherence to study intervention and
protocol
● Complete follow-up

43. COHORT STUDY
In a COHORT STUDY, a group of individuals
that is exposed to a risk factor (study group) is
compared with a group of individuals not
exposed to the risk factor (control group).

44. ● The researcher follows both groups over time
● Compares the occurrence of the problem related to the
risk factor in the two groups
● Determines whether a greater proportion of those with
the risk factor are indeed affected

45. CONCEPT OF COHORT
The term Cohort is defined as group of
people who share a common characteristic
or experience within a defined time period
e.g., age, occupation, exposure of a drug or
vaccine, birth cohort and marriage cohort
etc.

46. Distinct features of cohort study
●The cohort are identified prior to the appearance of
disease under study.
●The study groups so defined observed over a period of
time to determine the frequency of the disease among
them
● The study proceeds forward from cost to effect

47. A
C D
B
Disease +
Risk Factor -
Risk Factor +
Disease -
2 x 2 Contingency Table for Cohort Study
Total
Exposed
A+B
C+D
Incidence Among Exposed
A/(A+B)
Incidence Among Unexposed
B/(B+D)

48. Elements of a cohort study
● Selection of the study group
● Obtaining data on exposure
● Selection of the comparison group
● Follow up
● Analysis

49. Strengths of cohort studies
●Know that predictor variable was present before
outcome variable occurred (some evidence of causality)
● Directly measure incidence of a disease outcome
●Can study multiple outcomes of a single exposure (RR is
measure of association)

50. Weaknesses of cohort studies
● Expensive and inefficient for studying rare outcomes
● HERS vs. WHI
●Often need long follow-up period or a very large
population
● CARDIA
● Loss to follow-up can affect validity of findings
● Framingham

51. Selection of the study group
● General population
●Selected group of population e.g.; doctors, teachers,
nurses, school children etc.

52. Exposure Data
● Personal interview
● Mailed questionnaire
● Review of records
● Medical examination
● Environmental survey

53. Follow
up
● Periodic examination of each member of the cohort
● Reviewing physician and hospital record
● Routine surveillance of death records
● Mailed questionnaire
● Telephone calls
● Periodic home visit (on annual basis)

54. Follow up can be done through
• Mail • Email
• Online
Email

55. Interviews may be conducted
● Face-to-face
• Over the Telephone

56. Analysis
● Relative Risk
● Attributable Risk
● Odds ratio

57. Relative Risk (RR)
● Ratio of incidence of the disease (or death)
among exposed and the incidence among non-
exposed.
● It is a direct measure (or index) of the “strength”
of the association between suspected cause and
effect

58. Odds Ratio
(OR)
□ Measure of the strength of the association between risk
factor and outcome.
□ The derivation of the Odds Ratio is based on three
assumptions:
- the disease being investigated must be relatively rare
- the cases must be representative of those with the
disease
- the controls must be representative of those without
disease

59. Analysi
s
● Relative risk (RR)
Ie
RR = -----
Io
● Attributable risk (AR)
Ie-Io
AR = -------- X100
Ie
• Whereas
Ie : Incidence among exposed
Io : Incidence among non
exposed

60. Difference between BIAS and
CHANCE
● BIAS
It is deviation of results, or inferences from the truth
or processes leading to such deviation. It is a
systematic error
● CHANCE
It is a random error and may account for an apparent
association and make it appear real when it is not
(Type I or alpha error). It may lead to an association
being overlooked or missed when it truly exists (Type
II or Beta error)

61. Comparison
• Case control
Retrospective
Hospital based
Quick
• Easy to conduct
Small sample size
Less expensive
Rare diseases
None
• Cohort
Prospective/longitudinal
Community based
• Time consuming
Logistically difficult
Large sample size
Very expensive
Common disease
Incidence

62. Case Control VS Cohort
Study
● Factors
● Present
● Absent
● Risk Factors
● Exposed
● Unexposed
● Disease
● Present (Cases)
● Absent (Controls)
● Disease
● Present on Followup
● Absent on Followup
CASE CONTROL STUDY
COHORT STUDY
TIME LINE

63. INTERVENTIONAL STUDIES

64. INTERVENTIONAL STUDIES
●In Intervention Studies the researcher manipulates
the situation and measures the effects of this
manipulation.
●Usually (But not Always) 2 Groups are compared, one
in which the Intervention takes place and the other
group that remains “Untouched”

65. INTERVENTIONAL STUDIES
The 2 categories of Intervention Studies are:
1. Experimental Studies
2. Quasi-Experimental Studies

66. Experimental Studies
Individuals are randomly allocated to atleast 2 groups.
One group is subject to Intervention or Experiment
while the other group is not. Then the outcome of the
intervention is obtained by comparing the 2 groups

67. Diagram of Experimental
Study
Study Group
(Experimental)
1stData Collection
(Before Intervention)
Intervention
Last Data Collection
(After Intervention)
Study Population (Sampling)
Sample Population (Randomization)
COMPARE
Control Group
(Comparison)
1st Data Collection
(Same Time)
No Intervention
Last Data Collection
(Same Time)

68. Quasi-Experimental Studies
In this at least one characteristic of a true
experiment is missing. This may be Either:
● Missing of Randomization
● Missing of separate Control group
This however always includes manipulation of
independent variable that serves as intervention

69. Diagram of Quasi Experimental Model
Study Group before Intervention Study Group After
COMPARE
Control Group Before Control Group After

70. THANK
YOU

72. What is Incidence
• related event in a given population in a given
time
•X 1000
● Total Population at risk in a given time
● No. of new cases of a disease or health

73. What is Prevalence
● No. of new as well as Old cases of a disease
or health related event in a given
population in a given time
● Total Population at risk in a given time X 1000

74. What does Incidence Signifies
●It shows the RATE at which new diseases or health
problems occur in a population.

75. ●Prevalence shows the proportion of a population at
risk which is affected by a disease at a specific point in
time.
● It is further of 2 types
● Point Prevalence
● Period prevalence.

76. Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Case 9
Case 10
1Jan,2004 28 Dec,2004
Total No of
Patients
admitted
during this 1
year = 100

77. ●What is the prevalence of Hepatitis B on 1stJanuary
2004.
●What is prevalence of Hepatitis B during the year
2004.

78. Relation between Incidence and Prevalence
PREVALENCE
● Prevalence = Incidence x Duration of the
Disease
INCIDENCE
RECOVERY DEATH

79. Variations in Incidence and Prevalence
●Since Incidence depends on the occurrence of new
cases of a disease, a DECREASE in Incidence may be
due to
● Enhanced Resistance to the disease
● A change in Disease Etiology
● An effective prevention program that reduces exposure
to a known risk factor for the disease.

80. ● A DECREASE in Prevalence may be due to
● A decrease in Incidence
● A shorter duration of the disease due to either
improved treatment methods leading to more rapid
recovery or an increase in virulence leading to more
rapid death.

81. Risk Factor and Causality
● What is a Risk Factor
● A condition, physical characteristic, or behaviour that
increases the probability that a currently healthy
individual will develop a particular disease.

82. ●A Risk Factor may be a causal factor of the disease in
question or merely a marker for the increased
probability of disease.
●E.g while poor antenatal acre and drug use constitute
causal factors for neonatal mortality, socioeconomic
status would be considered a marker for neonatal
mortality.

83. Risk Assessment
●A number of epidemiological research designs are
used to evaluate the association between a disease
and a suspected risk factor.

84. Types of Epidemiological
Studies
● Descriptive Studies
● Also termed as Cross-sectional studies they determine
the disease frequency or prevalence of a condition.
Surveys are one example
● Analytic Studies
● Observational Studies
● Experimental Studies

85. Analytical Studies
● Observational
● Case Control Studies
● Cohort Studies
● Prospective Cohort Studies
● Retrospective Cohort Studies.

86. □Case control studies: Case-control studies are
those in which persons with a specified condition
(the cases) and pesons without the condition (the
controls) are selected for study. The proportion of
cases and controls with certain characteristics or
exposure is then measured and compared. For
example, knowing that there are 10 school
children with purple spots in grade 3, a set of
other third grade children from the same school
but without purple spots would be identified as
controls, and analysis done to see what different
exposures the purple-spotted children had than
the non-spotted

88. ●Cohort studies: groups of individuals with some
common feature (age and geography, for example)
are identified for study over time to learn about
differing health and illness experiences. For
example, one might enroll in a study all third
graders in a school and follow them until
graduation, attempting to identify the differences
in experiences of those who maintained a body
weight close to recommended and those who did
not.

92. EXPERIMENTAL STUDIES
● Quasi-Experimental Studies
● True Experimental Studies
● Randomization
● Control Group

93. Excercise
●In 1998 an outbreak of Cholera in a Peshawar
Suburb shows the following Data.
Cases presenting
with loose motions,
vomiting of acute
duration in the last 48
hours
People in the same
area who didn’t have
gastrointestinal
symptoms
135 55
Using community
water supply
Using own dugged
wells
2 103

94. ● From 1960 to 1992, 35250 adults aged 20-25 years were followed
up for habits of smoking and were assessed for presence of
Bronchogenic Carcinoma with following results.
CT Scan evidence of
Ca Lung
No evidence of ca
Lung
People who started
and continued with
Smoking
435 20165
People who didn’t
Smoke
10 14640