BASIC PRINCIPLES OF EPIDEMIOLOGY.pptx

LEARNING OBJECTIVES
› At the end of this lecture, the students of the 4th
year MBBS will be able to understand:
– Introduction.
– Modern epidemiology
– Definitions and their components.
– Aims of epidemiology
– Uses of epidemiology
– Scope of epidemiology.
– Basic Measurements in Epidemiology
– Tools of measurements
PROFESSOR DR AB RAJAR 2

EPIDEMIOLOGY

INTRODUCTION
› Epidemiology is the basic science of Preventive and
Social Medicine.
› Epidemiology is the scientific discipline of public
health to study diseases in the community to
acquire knowledge for the health care of society
including:
– Prevention
– Control
– Treatment

INTRODUCTION
Epidemiological principles and methods are applied
in —
• Clinical Research,
• Disease prevention,
• Health promotion,
• Health protection and
• Health services research.
The results of epidemiological studies are also used by
other scientists, including health economists, health
policy analysts, and health services managers.

MODERN EPIDEMIOLOGY
Infectious disease Epidemiology.
Chronic Disease Epidemiology.
Clinical Epidemiology.
Genetic Epidemiology.
Occupational Epidemiology.
Cancer Epidemiology.
Neuro-Epidemiology.

DEFINITION
› "The study of the distribution and determinants
of health-related states or events in specified
populations, and the application of this study to the
prevention and control of health problems ".
1 2 3
4 5
6

COMPONENTS OF DEFINITION.
› Study:
› Systematic collection, analysis, and interpretation
of data.
› Including observation, hypothesis testing, analytical
research, and experiments.
1

› Distribution:
› Refer to the analysis of an event by person, place &
time Epidemiology studies the distribution of
diseases it answers the question:
 Who, where, and when?
 Who? Where? When.
2

Epidemiology is concerned with the frequency and pattern
of health events in a population.
› A-Frequency:
› refers not only to the number of cases but also to the
relationship between the number of cases and the size of
the population
› B-Pattern:
› Refers to the occurrence of health-related events by time,
place, and person.
2

› Time patterns may be:
– annual,
– seasonal,
– weekly,
– or daily.
› Place patterns include:
– geographic variation,
– urban/rural differences,
– and location of work sites or schools.
› Personal characteristics such as age, sex, marital
status, and socioeconomic status, as well as behaviors
and environmental exposures.

› Determinant:
› It is any factor that influences health as chemical, physical,
social, biological, economic, genetic, or behavioral.
› Epidemiology is also used to search for determinants, which
are the causes and other factors that influence the
occurrence of disease and other health-related events.
3

› To search for these determinants, epidemiologists
use analytic epidemiology or epidemiologic studies
to provide the "Why?" and "How?" of such events.
Why? How?

› Health-related state or event :
› It is defined as anything that affects the well-being
of a population.
› As: disease, cause of death, behaviors, etc.
4

› Specified populations :
› Although epidemiologists and direct healthcare providers
(clinicians) are both concerned with the occurrence and
control of disease, they differ greatly in how they view "the
patient."
 The clinician is concerned about the health of an individual.
 The epidemiologist is concerned about the collective health of the
people in a community or population.
5

› Application of this study to the control:
› Epidemiological studies have direct and practical
applications for the prevention of diseases &
promotion of health Epidemiology is a science and
practice Epidemiology is an applied science.
6

BASIC CONCEPT
› Risk: The probability of having a bad outcome.
› Risk factors: A condition, physical characteristic,
or behavior that increases the probability that a
currently healthy individual will develop a particular
disease. "

BASIC CONCEPT
Modifiable risk factors:
› A risk factor that can be reduced or controlled by
intervention, thereby reducing the probability of
disease.
– As : (Physical inactivity, Tobacco use, AIcohoI use,
Unhealthy diets).
Non-modifiable risk factors:
› A risk factor that cannot be reduced or controlled
by intervention, for example, Age, Gender, Race, or
Family history (genetics).

ULTIMATE AIM OF EPIDEMIOLOGY
I. To eliminate or reduce the health problems of the
community.
II. To promote the health and well-being of society
as a whole.

AIMS & OBJECTIVES OF EPIDEMIOLOGY
To describe the distribution and magnitude of health and
disease problems in the human population.
To identify etiological factors (risk factors) in the pathogenesis
of the disease.
To provide data essential to the planning, implementation, and
evaluation of services for the prevention, control, and
treatment of disease and setting priorities among those
services.

SCOPE OF EPIDEMIOLOGY
Causation of the disease.
Natural history of the disease.
Health status of the population.
Evaluation of Interventions.

CAUSATION OF THE DISEASE.
› Most diseases are caused by interaction between
genetic and environmental factors. (Diabetes)
› Personal behaviors affect this interplay.
› Epidemiology is used to study their influence and
the effects of preventive interventions through
health promotion.

1-CAUSATION OF THE DISEASE.

2-NATURAL HISTORY OF THE DISEASE
› Epidemiology is also concerned with the course and
outcome (natural history) of diseases in individuals
and groups.

2-Natural History of the Disease

3-HEALTH STATUS OF THE POPULATION
 Epidemiology is often used to describe the health
status of the population.
 Knowledge of the disease burden in populations is
essential for health authorities.
 To use limited resources to the best possible effect
by identifying priority health programs for
prevention and care.

3- THE HEALTH STATUS OF THE POPULATION

4. EVALUATION OF THE INTERVENTIONS
 To evaluate the effectiveness and efficiency of
health services.
 This means determining things such as —
 Impact of Contraceptive use on Population Control.
 The efficiency of sanitation measures to control diarrheal
diseases and
 The impact of reducing lead additives in petrol.

4-EVALUATION OF INTERVENTIONS

CLINICAL EPIDEMIOLOGY
› Applying epidemiological
principles and methods to
problems encountered in
the practice of medicine
has led to the
development of-

APPLICATIONS OF EPIDEMIOLOGY IN PUBLIC
HEALTH
I. Preventing disease and promoting health.
II. Community health assessment (Community
Diagnosis) and priority setting.
III.Improving diagnosis. treatment and prognosis of
clinical diseases.
IV.Evaluating health interventions and programs.

EPIDEMIOLOGY AND PUBLIC HEALTH
› Public health, refers to collective actions to improve
population health.
› Epidemiology, one of the tools for improving public
health, is used in several ways.

EPIDEMIOLOGY & CLINICAL MEDICINE
› 1. In Clinical Medicine the unit of study is a 'case',
but in Epidemiology the unit of study is a 'defined
population ' or 'population at risk'.
› Physician is concerned with the disease in the
individual patient, whereas Epidemiologist is
concerned with the disease pattern in the entire
population.
› So, Epidemiology is concerned with both the Sick &
Healthy.

› 2 In Clinical Medicine, the physician seeks to
diagnose for which he derives a prognosis and
prescribes specific treatment.
› The Epidemiologist is confronted with the relevant
data derived from the particular epidemiological
study. (Community Diagnosis)
› He seeks to identify the source of infection, mode
of transmission, and an etiological factor to
determine future trends, prevention and control
measures.

› 3. In Clinical Medicine patient comes to the Doctor.
› Epidemiologist, goes to the community to find out
the disease pattern and suspected causal factors in
the question.

EPIDEMIOLOGICAL APPROACH.
l. Asking
questions.
2. Making
Comparisons.

1. ASKING QUESTIONS
RELATED TO HEALTH
EVENTS
› What is the event? (Problem)
› What is magnitude?
› Where did happen?
› When did happen?
› Who is affected?
› Why did it happen?
RELATED TO HEALTH
ACTION
› What can be done to reduce
the problem?
› How can be prevented in the
future?
› What action should be taken
by the community?
› What resources are required?
› How are activities to be
organized?
› What difficulties may arise?
Epidemiology is "a means of learning by asking questions and
getting answers that lead to further questions."
PROFESSOR DR AB RAJAR
37

These questions can be referred to as:
Case definition [what]
Person [who]
Place [where]
Time [when]
Causes [why]

2. MAKING COMPARISONS
› To find out the differences in the AGENT, HOST, and
ENVIRONMENT conditions between the two groups.
› Weighs. balances and contrasts give clues to
ETIOLOGICAL HYPOTHESIS.

Basic Measurements in
Epidemiology

DEFINING HEALTH AND DISEASE.
› Definition
• “Health is a state of complete physical, mental,
and social well-being and not merely the absence
of disease or infirmity "
(WHO in 1948)

› This definition — criticized because of the difficulty
in defining and measuring well-being — remains an
ideal.
› The World Health Assembly resolved in 1977
that all people should attain:
• level of health permitting them to lead socially and
economically productive lives by the year 2000.
(Health for All by 2000)

› Practical definitions of health and disease are
needed in epidemiology, which concentrates on
aspects of health that are easily measurable and
amenable to improvement.
› Definitions of health states used by epidemiologists
tend to be simple, for example,
– "disease present" or "disease absent'

› There is often no clear distinction between normal
and abnormal.
› Especially, for normally distributed continuous
variables that may be associated with several
diseases. •
 Examples:
 Cut of point for Blood Pressure- HTN.
 Cut of point of Hemoglobin- Anemia.
 Normal Range of Blood Cholesterol.

Serum Cholesterol (mg%) Frequency
125-135 5
135-145 22
145-155 25
155-165 130
165-175 140
175-185 260
185-195 274
195-200 282
200-210 268
210-220 270
220-230 135
230-245 135
245-255 24
255-265 24
265-275 8
Total 2000

MEASURING DISEASE FREQUENCY

INCIDENCE AND PREVALENCE
 These are fundamentally different ways of
measuring disease frequency.
 The incidence of disease represents the rate of
occurrence of new cases arising in a given period
in a specified population,
while
 Prevalence is the number of existing cases
(old+ new) in a defined population at a given point
in time.

INCIDENCE
› "Number of new cases occurring in a defined
population during the specified period of
time"
 Incidence = Number of new cases during a given period
/ Population at risk x 1000.

SPECIAL INCIDENCES
•Rate at which acute
disease is
spreading.
Attack Rate
• % of exposed persons
developing disease after
primary case exposure.
Secondary
Attack Rate

PREVALENCE
› Prevalence is the total no of existing cases ( old
+ new) in a defined population at a particular point
in time or specified period.
 Prevalence = Total no of cases at a given point in time
/ Estimated population at time x 100

RELATION BETWEEN INCIDENCE & PREVALENCE
› Prevalence = Incidence x Mean duration of d/se.
• P= I x D
› Example— If
• I=10 cases per 1000 per year.
• D = 5 years.
– P= 10 ᕁ 5
50 cases per 1000 population

TYPES OF PREVALENCE
1. Point Prevalence:
•Prevalence for a given point of time.
2. Period Prevalence:
•Prevalence for a specified period.

Activity

Cases of cold infections in class 4th : Class
size = 20
January February March
› What is the period prevalence during February?
› What is the point prevalence on the 28th of February?
› What is the incidence in February?

SOLUTION
What is the period prevalence during February?
– 6/20=30.00%
What is the point prevalence on the 28th of
February?
– 1/20=5.0%
What is the incidence in February?
– 4/18=22.20%

FACTORS INFLUENCING THE PREVALENCE
INCREASED BY
• Longer duration of the disease
• Prolongation of life of patients without
cure
• Increase in new cases (increase in
incidence)
• In-migration of cases
• Out-migration of healthy people.
• In-migration of susceptible people
• Improved diagnostic facilities
• (better reporting)
DECREASED BY
• Shorter duration of the disease
• High case-fatality rate from the disease
• Decrease in new cases (decrease in
incidence)
• In-migration of healthy people
• Out-migration of cases
• Improved cure rate of cases

SCOPE OF MEASUREMENTS
IN EPIDEMIOLOGY

MEASUREMENTS IN EPIDEMIOLOGY
i. Measurement of mortality.
ii. Measurement of morbidity.
iii. Measurement of disability.
iv. Measurement of natality.
v. Measurement of presence or absence of attributes.
vi. Measurement of health care need.
vii. Measurement of environmental & other risk factors.
viii.Measurement of demographic variables.

TOOLS OF
MEASUREMENTS

TOOLS OF MEASUREMENTS
Rate
Ratio
Proportion
› Basic tools are -
› Used for expression of disease magnitude

RATE…..
› RATE:
› A rate measures the occurrence of some particular
event in a population during a given time period.
–A rate comprises of following four elements:
› Numerator.
› Denominator.
› Time specification.
› Multiplier, (E.g. death rate).

NUMERATOR
› Numerator refers to the number of times an event (
e.g. sickness, birth, episodes of sickness) has occurred
in a population, during a specific time period.
› Note: Numerator is a component of the denominator in
calculating a rate, but not in a ratio.

DENOMINATOR
› Numerator has little meaning unless it is related to the
denominator; therefore epidemiologists has to choose
an appropriate denominator while calculating a rate.
› It may be:
– Related to the population or
– Related to the total events.

DENOMINATOR
› Related to the population:
– Mid-year population
– Population at risk
– Person-time
– Person – distance
– Sub-groups of the population
› Related to the total events:
– Eg : number of accidents per 1000 vehicle owners

POPULATION AT RISK:
› Population at risk:
› Portion of a population that is susceptible to a
disease
› Can be defined on the basis of demographic or
environmental factors Prevalence.
› Population at risk: Examples:
– Population at risk of developing carcinoma of the cervix: Female
population Age > 30 and < 70 years.
– Population at risk of hepatitis B Those individuals anti-HBc negative
Prevalence

TYPES OF RATE….
 DEATH RATE:
–Death Rate =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑖𝑛 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
x 1000.
–Various categories of rate are:
a. Crude Death Rate.
b. Specific death rate.
c. Adjusted or standardized Rates.

CRUDE DEATH RATE:
– CRUDE DEATH RATE:
› These are actual observed rates such as the birth rate
& death rates.
› The denominator is whole population.
› Known as Unstandardized rate ( E.g. Crude Birth Rate).
– CBR=
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜 𝑜𝑓 𝑙𝑖𝑣𝑒 𝑏𝑖𝑟𝑡ℎ 𝑎𝑡 𝑎 𝑝𝑙𝑎𝑐𝑒 𝑖𝑛 𝑎 𝑦𝑒𝑎𝑟
𝑇𝑜𝑡𝑎𝑙 𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑝𝑙𝑎𝑐𝑒 𝑦𝑒𝑎𝑟
x 1000.
› Note: Population of 1st January is less & at 30th
more, so we take mid-year ( 1st July) population.

SPECIFIC RATES:
› These are specific rated due to:
–Specific causes (e.g. Tuberculosis) or
–Occurring in specific groups (age or sex groups )
or
–During specific time period (e.g. annual, monthly
or weekly rates).
› So, in this denominator is a specific segment of
population e.g., some specific rates are:
–IMR
–MMR
–Neonatal Mortality rate.

CASE FATALITY RATE
› Percentage of particular cases dying during a
particular disease epidemic.
› Killing power of disease, particularly acute diseases.
–CFR=
𝑁𝑜 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ 𝑑𝑢𝑒 𝑡𝑜 𝑐ℎ𝑜𝑙𝑒𝑟𝑎
𝑇𝑜𝑡𝑎𝑙 𝑁𝑜 𝑜𝑓 𝑐ℎ𝑜𝑙𝑒𝑟𝑎 𝑐𝑎𝑠𝑒𝑠
ᕁ100

PROPORTIONAL MORTALITY RATE
› Proportion of % of death due to particular cause
out of total deaths.
– It measures the disease burden.
Under-5,Proportional mortality=
𝑁𝑜 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ 𝑏𝑒𝑙𝑜𝑤 5 𝑦𝑒𝑎𝑟𝑠
𝑇𝑜𝑡𝑎𝑙 𝑁𝑜 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠
ᕁ100

SURVIVAL RATE
› Percentage of the treated
patients remaining alive at the
end of 5 years of treatment.
› Yard stick for assessing the
standing of therapy in
cancer.
– Survival Rate=
𝑝𝑡𝑠 𝑎𝑙𝑖𝑣𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑒𝑛𝑑 𝑜𝑓 5 𝑦𝑟𝑠
𝑇𝑜𝑡𝑎𝑙 𝑁𝑜 𝑜𝑓 𝑝𝑡𝑠 𝑡𝑟𝑒𝑎𝑡𝑒𝑑
ᕁ100

ADJUSTED OR STANDARDIZED RATES
› These are obtained by the
direct and indirect method
of standardization or
adjustment – age & sex
standardized rates.

COUNT:
› Count No. of new AIDS cases:
–City A 58
–City B 35

DIVIDE
No. of new AIDS cases
No. Year Population
City A 58 2004 25,000
City B 35 2004-2005 7,000
• City A: 58/25,000/ 1 year
• City B: 35/7000/ 2 years Divide

RATIO
› It expresses a relation in size between two random
quantities.
› Ratio is the result of dividing one quantity by
another.
› It is expressed in the form of x/y or x:y.
› In ratio, the numerator is not a component of the
denominator.

RATIO Cont….
› The ratio of white blood cells relative to red blood
cells is 1;600, meaning that for each WBC there are
600 RBSs.
–Others as:
–Sex-ratio i.e. male to female ratio.
–Doctor-population ratio.
–Child-women ratio.

RATIO.
› In a class that has 20 male students and 80 female
students.
› We can think about this in several ways.
› The ratio of boys to girls as 20:80 or 20/80.
› Aa 1:4 ratio (or 1/4 ratio). [This indicates that for every
boys, there are four girls].
› From the inverse perspective, i.e., the number of girls
relative to the number of boys; in this case the ratio of girls
to boys is 80/20 which is equivalent to 4 to 1, i.e., there are
four girls for every boys.

PROPORTION
› A proportion is a ratio, which indicates the relation
in the magnitude of a part of the whole.
• The numerator is always included in the denominator.
• A proportion is usually expressed as a percentage (%)
e.g.:
P=
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜 𝑜𝑓 𝑆𝑐𝑏𝑖𝑒𝑠 𝑎𝑡 𝑎 𝑐𝑒𝑟𝑡𝑖𝑎𝑛 𝑡𝑖𝑚𝑒
𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐ℎ𝑖𝑙𝑑𝑟𝑒𝑛 𝑖𝑛 𝑡ℎ𝑎𝑡 𝑎𝑟𝑒𝑎 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒.
x 100.

PROPORTIONS
› The mortality from bird flu was 44 from total number of
infected 123 persons in last year in China.
› In total, 123 people were infected, and 44 of these died.
› Therefore, the proportion who died was 44/123, which could
be expressed as a decimal fraction (0.36) or as a
percentage (36%).
› This proportion is referred to as the "case-fatality" rate,
although strictly speaking, it is a proportion and not a rate.

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