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Introduction
to
Epidemiology
Dr.Fatima Alkhaledy
M.B.Ch.B;F.I.C.M.S/C.M.
Outlines
 History of Epidemiology.
 Definition of Epidemiology and its components.
 Epidemiological Basic concepts.
 Aims of Epidemiology.
 Ten Uses of Epidemiology.
 Scope or The Areas of Application .
 Types of Epidemiological Studies.
Epidemiology
The term epidemiology derived from Greek words :
Epi
Among
Demos
People
Logos
Study
History of epidemiology
 The history of epidemiology has
its origin in the idea goes back to
(400 B.C)
 Hippocrates Who suggest that
environment & human behaviors
affects health.
History of epidemiology cont.
 1662 – John Graunt, who
published a landmark analysis
of Mortality data.
 It was the first quantify
patterns of birth, death, and
disease occurrence.
History of epidemiology cont.

1800 – William Farr work by
systematically collecting and
analyzing Britain's mortality
statistics.

Farr, considered the father of
modern vital statistics and
surveillance, developed many of
the basic practices used today in
vital statistics and disease
classification.
History of epidemiology cont.



1854 -John Snow , was considered the “father of
field epidemiology.
John Snow was conducting a series of
investigations.
Twenty years before the development of the
microscope, Snow conducted studies of cholera
outbreaks both to discover the cause of disease and
to prevent its recurrence.
History of epidemiology cont.
 19th and 20th centuries
epidemiological methods
applied in investigation of disease occurrence , at
that time, most investigators focused on acute
infectious diseases.
 1930s &1940s non-infectious diseases were also
investigated for their risk-factors, (as lung cancer to
smoking)
History of epidemiology cont.
 1980s -extended to the studies of injuries and
violence.
 Beginning in the 1990s after the terrorist attacks of
September 11-2001, epidemiologists have had to
consider not only natural transmission of infectious
organisms but also deliberate spread through
biologic warfare and bioterrorism
Is a study of distribution & determinants of healthrelated states or events in specified populations, and
the application of this study to the control of health
problems.
Components of the definition
1
2
3
Is a study of distribution & determinants of health4
5
related states or events in specified populations, and
6
the application of this study to the control of health
problems.
Components of the definition cont.
1
Study:
Systematic collection, analysis and
interpretation of data.
Including : observation ,hypothesis testing
, analytical research and experiments.
Components of the definition cont.
2
Distribution :
 Refer to analysis of an event by person, place & time
 Epidemiology studies distribution of diseases it answers
the question:
who, where and when?
Who?
Where?
When?
Components of the definition cont.
2
Epidemiology is concerned with
the frequency and pattern of health events in a
population.
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
A.
Components of the definition cont.
Pattern:
Refers to the occurrence of health-related events by time,
place, and person.
B.
o
Time patterns may be annual, seasonal, weekly, daily.
o
Place patterns include geographic variation, urban/rural
differences, and location of work sites or schools.
o
Personal characteristics such as age, sex, marital status,
and socioeconomic status, as well as behaviors and
environmental exposures.
Components of the definition cont.
3
Determinant:
It is any factor that influence health as : chemical ,
physical, social , biological , economic , genetic,
behavior.
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.
Components of the definition cont.
To search for these determinants, epidemiologists use
analytic epidemiology or epidemiologic studies to provide
the “Why?” and “How?” of such events.
?
Why?
HOW?
Components of the definition cont.
4
Health-related state or event :
It is defined as anything that affects the well-being of
a population.
As : disease, cause of death, behaviors ,ect…
Components of the definition cont.
5
Specified populations :
Although epidemiologists and direct health-care
providers (clinicians) are both concerned with
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.
Components of the definition cont.
6
Application of this study to the control:
Epidemiological studies have direct and
practical applications for prevention of
diseases & promotion of health Epidemiology
is a science and practice Epidemiology is an
applied science
Epidemiology is concerned with:
Distribution and determinants of health and
diseases
 Morbidity
 Mortality
 Disability
Morbidity & Mortality
 Morbidity: refers to the state of being
diseased or unhealthy within a population.
 Mortality : refers to the number of people
who died within a population.
Morbidity
It is the Incidence of disease , it may cause
Death
not cause death
By another meaning : mortality refers to the state of
being diseased or unhealthy within a population
Note :High morbidity rate means (vary common disease)
as a diabetes , hypertension .
Mortality
It is the measurement of death .
As HIV , Ebola virus disease .
Note : High mortality rate means more
incidence of death.
Example
mortality &
morbidity
That means disease is vary
common and incidence of
death is vary high.
As: acute pencrititis
mortality &
morbidity
That means disease is un
common and incidence of
death is low.
As : vitiligo
Example
mortality &
morbidity
mortality &
morbidity
That means disease is vary
common and incidence of
death is vary low .
That means disease is un
common and incidence of
death is high .
As : Diabetes
As: Amyotrophic lateral
sclerosis (ALS)
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. “ p
value ≤ 0.05 “
Basic concept cont.
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 ,Alcohol use ,
Unhealthy diets) .
Non-modifiable risk factors:
A risk factor that cannot be reduced or controlled by
intervention, for example: Age , Gender , Race , Family
history (genetics).
Basic concept cont.
Causality:
A cause effect relationship.
In epidemiology, the cause is the exposure and the
effect is disease or death( criteria of causation )
Criteria of causation:
1- Temporality: the risk factor come before the disease.
2- Consisting: the result of the study should be comparable
to the results of other studies.
3- Dose response: when the risk factor increase the disease
should increase also vise versa.
4- Study design.
5- Degree of association.
Aims of Epidemiology
 Epidemiology has three main aims:
1. To describe disease patterns in human
populations.
2. To identify the causes of diseases (also known
as etiology).
3. To provide data essential for the management,
evaluation and planning of services for the
prevention, control and treatment of disease.
ten Uses of Epidemiology
 Because most uses of epidemiology need not be
restricted to human populations, the expression
host" is preferred to "the host" .
"a
 In addition, the term "disease" includes injury and it may
involve a consideration of sequelae such as disability,
defect and impairment.
ten Uses of Epidemiology cont.
Epidemiological Triangle
1) To determine which in
the three possible sets of
disease factors, host, agent
and environment, are
important in the occurrence
of a specific disease or class
of diseases, and to explore
the manner in which they
interact.
ten Uses of Epidemiology cont.
Agent TB
organism
 Agent, or microbe that
causes the disease (the
“what” of the Triangle)
 Host, or organism
harboring the disease
(the “who” of the
Triangle)
 Environment, or those
external factors that
cause or allow disease
transmission (the
“where” of the Triangle)
TB
Host
Poor nutrition
Concurrent
disease
Low immunity
Environment
Crowding
Poor
ventilation
Bad sanitation
ten Uses of Epidemiology cont.
2) To study the occurrence of disease in a population for
purposes of community diagnosis and prognosis.

A clinician sees a disease as it occurs in an individual.

The epidemiologist is interested in the individual too, but
his attention is primarily directed toward the group.
ten Uses of Epidemiology cont.







The severity of disease in a community can be gauged by
several indices:
whether it is a leading cause of death;
whether it has a high case fatality or other measure of
complications;
whether it involves a large proportion of the young;
whether it leaves permanent disability, defect or
impairment;
whether it adversely affects the economic status of a
community.
The role of the epidemiologist can be described as that of a
community diagnostician.
ten Uses of Epidemiology cont.
3) To describe the epidemiology of a disease or class of
diseases.

Facts or events relating to the occurrence of a disease
(or class of diseases) constitute its epidemiology.

The occurrence of a disease and its severity and outcome
are resultants of a complex of interacting factors, some
of which are host factors and others, agent and/or
environmental.
ten Uses of Epidemiology cont.

A description of the epidemiology of a disease involves a
thorough study of that disease in all its aspects including
its frequency in various population, and known and
suggested factors in the premorbid, morbid and postmorbid periods.
 The premorbid period is the period preceding the
inception of a disorder.
 The morbid period, the pathological and clinical
course .
 The post-morbid period, the aftermath.
Natural history of health event
ten Uses of Epidemiology cont.
4) To measure risk.
 The epidemiologist may use rates which can show the probability that
could make :
 A person acquire a particular disease during a given period of time (attack
rate, morbidity rate, case incidence rate).
 A person have a particular disease at a given point in time (prevalence
rate).
 A person die of a particular disease during a given period of time
(mortality rate, death rate); or if he has a particular disease, that he will
die of it during a given period of time (case fatality rate).

Such probabilities are measures of risk and they are useful to know
prognosis.
ten Uses of Epidemiology cont.
5) To study the occurrence of disease or death with time as a
variable. Such a study is referred to as a historical study.

In historical studies, it often is convenient to speak of trends,
namely, secular trends and cyclic trends.
A secular trend ranges over a long period such as a century, and
portions of it may increase, decrease or remain stationary.
A cyclic trend must exhibit periodic fluctuations regardless of its
duration either long or short
ten Uses of Epidemiology cont.
6) To aid in the search for causes of disease.

In general, a cause can be defined as an agent or any
host or environmental factor that is influential in
producing disease or accelerating its appearance.
ten Uses of Epidemiology cont.
7) In disease prevention and control.
 The epidemiologist helps to control a disease when he
identifies it, describes its epidemiology, demonstrates its
existence in a community and investigates its source.
Primary control or disease prevention is the ideal control measure, it aims to
Prevent the initial development of the disease.
Secondary control consists of diagnosis and treatment, it aims to reduce the
severity and prevent complications.
Tertiary control involves rehabilitation of the disabled and correction of
defect or impairment.
ten Uses of Epidemiology cont.
8) To aid in the identification of clinical syndromes.

A syndrome is a group of signs and symptoms that
occur together and characterize a disease.
ten Uses of Epidemiology cont.
9) To aid in the detection of pre-symptomatic and latent
disease.

Many diseases such as tuberculosis, diabetes, hypertension, g
cancer of the cervix, neoplasms of the lung can be detected
before the onset of symptoms or the development of serious
changes.

The way to accomplish this is through mass screening
techniques , pre-employment physical examinations, periodic
health examinations, special surveys and routine
examinations in hospitals and clinics.
ten Uses of Epidemiology cont.
10) In administrative medicine and operations research.

One might evaluate the success of a disease control
program; the needs of a hospital in terms of equipment,
personnel and space; the quality of medical care in a
community; and the efficiency of hospital administration
Scope of Epidemiology

Strictly Speaking, There Is No Life Science, Where
Epidemiological Approach And Principles Cannot Be
Applied.
From Womb To Tomb Epidemiology Is
Applicable
Types of epidemiological
Studies

Study design: is defined to be the process of
planning an empirical investigation to assess a
conceptual hypothesis about the relationship
between one or more exposure and a health out
come
Types of epidemiological Studies
cont.
1.


Observational studies:
Descriptive study : ( cross sectional )
Analytic study : (cohort , case control)
2. Experimental studies:
•
Community trails
•
Clinical trails
Study design
Observation
Experimental
( comparison group)
NO
YES
Descriptive Analytical
Non-randomized
Clinical trial” “ Community trial
Randomized
Cross-sectional
Cohort Case – control
Descriptive study
(Cross –sectional study)
 Cross-sectional is : the collection of data from more
than one case at one moment in time.
 (Examines relationship between exposure and outcome
prevalence in a defined population at a single point in
time).
 In cross sectional studies individuals are classified as
diseased and disease free and exposed and not exposed
to the risk factor at single point of time.
(Cross –sectional study) cont.

It helps to know:
1.Prevalence rate
2.Risk factor
 It is
The basics for further studies.
 It provides association not causation.
(Cross –sectional study) cont.
(Cross –sectional study) cont.
Advantages:
*Less time-consuming
than case-control or
cohort studies.
*Inexpensive.
*Good, quick picture of
prevalence of exposure
and prevalence of
outcome.
Disadvantages:
*Difficult to determine
temporal relationship
between exposure and
outcome (lacks time
element).
*Not suitable for rare
disease.
*Unable to measure
incidence.
Analytical Studies
 A method for testing a hypothesis as part of an
investigation of the association between a disease and
possible causes of that disease) .

They describe association between exposure and
outcome.
Risk factor
Disease
Cohort studies
 A cohort study typically examines multiple health effects
of an exposure; subjects are defined according to their
exposure levels and followed over time for outcome
occurrence .
 The rates of disease incidence among the exposed and
unexposed groups are determined and compared.
 Measure of association “ relative risk”.
Cohort studies cont.
Presen
t
Future
Cohort studies cont.
1. Prospective cohort :
outcome occurs after the
study is initiated.
 Time consuming,
expensive
 More valid information
on exposure.
 Inefficient for rare
diseases
Cohort studies cont.
2.Retrospective cohort study:
 Outcome has already occurred at study
initiation (cohort of exposed & unexposed are
assembled from past medical records and
followed forward to determine present
incidence of disease).
 Cheaper and faster than a prospective study
 Requires good historical exposure data on
subjects(accurate records).
 Looks back in time to study events that have
already occurred .
Retrospective cohort study
Measure
exposure
and
confounder
variables
Exposed
Outcome
Non-exposed
Outcome
Time
Study begins here
Cohort studies cont.
Advantages:

Can evaluate multiple effects of a single exposure

More efficient for rare exposures and outcomes with long induction and
latency periods

Can directly measure incidence

Clear chronological relationship between exposure and outcome
Disadvantages:
• Expensive
• Time-consuming
• Inefficient for rare diseases with long latency.
• Loss to follow-up
Case-Control Studies
 Examines multiple exposures in relation to an
outcome; subjects are defined as cases and controls,
and exposure histories are compared.
 First, identify the cases and the controls. Then, look
back in time to learn which subjects in each group
had the exposure(s).
 Efficient
for rare diseases.
 Measure of association “odds ratio “.
Case-Control Studies
Exposure present
cont.
Cases
(disease
present)
Exposure absent
Study
population
Exposure present
Controls
( disease
absent)
Exposure absent
PAST
TIME
Study begins here
PRESENT
Case-Control Studies
Advantages:
*Relatively inexpensive
*Less time-consuming than
cohort studies.
*Can evaluate effects of
multiple exposures.
*Efficient for rare outcomes
or outcomes with long
induction or latency periods.
cont.
Disadvantages:
*Subject to recall bias (based
on subjects’ memory and
reports).
*Inefficient for rare
exposures (such as
*Difficult to establish clear
chronology of exposure and
outcome).
Case - control
Rare disease
Cohort
Yes
Rare exposure
Yes
Multiple outcome
Yes
Several risk
factors
Yes
Dose – response
relations
Disease with long
latent period
Yes
Yes
Experimental Studies
 Two types:
1- Controlled clinical trials (Randomized Control Trials)
 Emphasis on the individual
2- Community interventions
 Emphasis on the community
Experimental Studies cont.

There are three key components of an experimental
study design:
(1) pre-post test design .
(2) a treatment group and a control group .
(3) random assignment of study participants.
Randomization:

◦
Equal probability of participants to be assigned
to intervention or control groups
Experimental Studies cont.
Outcome
Intervention
Not
outcome
Study
population
Control
Time
Study begins here (baseline point)
Future
Outcome
Not
outcome
Experimental Studies cont.
Advantages:
*Best evidence study design.
*No inclusion bias.
*Controlling for possible
confounders.
*Comparable groups (using
randomization).
Disadvantages:
*Large trials.
*Long term followup (possible losses).
*Expensive.
*Possible ethical
questions.
CONFOUNDING
 Confounder: a factor that distorts the true
relationship of the study variables because of being
related to both the outcome and the exposure.
 Distorted estimating
of exposure effect results
because the exposure effect is mixed with the
effect of extraneous variable .
 Also called third
variable
CONFOUNDING cont.
Exposur
e
Association
Outcom
e
Association
Association
Confoundi
ng
variable
Associations of a hypothetical exposure, disease, and confounding
variable
CONFOUNDING cont.
Association
Smoking
Cancer
Association
Association
Pollution
Associations of smoking, cancer, and pollution
CONFOUNDING cont.

To be a confounding factor, two conditions must be
met-:
1- Be associated with exposure without being a result
of exposure.
2- Be associated with outcome independently of
exposure (not an intermediary).
CONFOUNDING cont.
 Confounding can be controlled by:
- Randomization: assures equal distribution of
confounders between study and control groups.
- Restriction: subjects are restricted by the levels of a
known confounder.
- Matching: potential confounding factors are kept
equal between the study groups.
- Stratification for various levels of potential
confounders.