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Principles of Epidemiology
Dr. Kashif Shafique
MBBS (Dow),MPH (Glasgow), PhD (Glasgow)
Professor & Principal
School of Public Health
Director – Office of Research, Innovation & Commercialization
Dow University of Health Sciences.

Today’s agenda
 Introductions
 Mine
 Yours
 Epidemiology

Agenda for this course
 Introduction to epidemiology
 Measures of morbidity and mortality
 Epidemiological research designs
 Ecological studies
 Cross sectional studies
 Case control studies
 Cohort studies
 Randomized control trials
 Estimation of risk
 Analysis of an association
 Alternative explanations of an association
 Causal inferences from epidemiological studies
 Application of epidemiology in early diagnosis
QUIZ
QUIZ
QUIZ

What to read
 Epidemiology 4th Edition
 Leon Gordis
 Assessment of this course
 30% Quiz
 70% final examination (Best Choice Questions & Short
Answer Questions)

Definitions of Epidemiology
Lilienfeld A: in Foundations of Epidemiology
The study of the distribution of a disease or a
physiological condition in human populations and of the
factors that influence this distribution
Last JM: A Dictionary of Epidemiology
The study of the distribution and determinants of health
related states and events in populations and the
application of this study to control of health problems“

Key Words in Definition
 Disease distribution - who, when, where
 Frequency, distribution, other factors generate
hypotheses about determinants
 A determinant is a characteristic that
influences whether or not disease occurs

1. Descriptive epidemiology
Describing patterns and trends in health
and disease in populations
2. Analytical epidemiology
Examining associations and causation
3. Experimental epidemiology
Testing population interventions
Types of epidemiology and their uses

Epidemiologic approaches
DESCRIPTIVE Health and disease in the community
What? Who? When? Where?
What are the
health problems
of the
community?
What are the
attributes of
these illnesses?
How many people
are affected?
What are the
attributes of
affected persons?
Over what
period of time?
Where do the
affected people
live, work or
spend leisure
time?
ANALYTIC Etiology, prognosis and program evaluation
Why? How?
What are the
causal agents?
What factors
affect outcome?
By what mechanism
do they operate?

“Epidemiology . . .
is a Greek word that means to put
people to sleep with charts and
graphs.”

 Time
 Place
 Person
Three essential characteristics of disease
that we look for in descriptive studies are...

Time
 Calendar Time
 Time since an event
 Age (time since birth)
 Seasonality

Person
 Age, gender, ethnicity
 Genetic predisposition
 Concurrent disease
 Diet, exercise, smoking
 SES, education, occupation

Place
 Geographic place
 presence or agents or vectors
 climate
 population density
 economic development
 nutritional practices
 medical practices

 Often makes use of routinely collected data,
e.g. death certification data, hospital
episode statistics, infectious disease
notifications
 May require special surveys
 Can’t answer ‘why?’ but can raise
hypotheses about causes
Descriptive epidemiology

(a) Mortality in middle age during the last 100 years in England and Wales.
Morris J N Int. J. Epidemiol. 2007;36:1165-1172
Published by Oxford University Press on behalf of the International Epidemiological Association ©
The Author 2007; all rights reserved.

Descriptive epidemiology: application
studying the frequency and
distribution of disease
to generate hypotheses
about causes
(the academic focus)
to inform local public
health action
(the service focus)

Epidemiologic Activities
 …are often framed under the mantle of descriptive and analytic
epidemiology
 Descriptive epidemiology – person, place & time
 Demographic distribution
 Geographic distribution
 Seasonal patterns etc.
 Frequency of disease patterns
 Useful for:
 Allocating resources
 Planning programs
 Hypotheses development

Blood pressure cases 'to top 1bn'
High blood pressure is out of control
around the world, with the number of
sufferers expected to exceed a billion
within 20 years, experts warn.
One in four adults already has the
condition, which increases the risk of
heart disease, stroke and death.
In the news

Descriptive epidemiology
+
Focus on cause and effect
=
“analytical epidemiology”

Three essential characteristics that are examined to
study the cause(s) for disease in analytic
epidemiology are...
 Host
 Agent
 Environment

Host
Environment
Agent
Epidemiologic Homeostasis

Host Factors
Personal traits
behaviors
genetic predisposition
immunologic factors
Agent
Host
Environment
• Influence the chance
for disease or its severity
The Epidemiologic Triangle

Agents
Biological
Physical
Chemical
Agent
Host
Environment
• Necessary for disease to
occur

Environment
External conditions
Physical or biologic
or social
Agent
Host
Environment
• Contribute to the disease
process

Epidemics arise when host, agent, and
environmental factors are not in balance
 Due to new agent
 Due to change in existing agent (infectivity,
pathogenicity, virulence)
 Due to change in number of susceptibles in
the population
 Due to environmental changes that affect
transmission of the agent or growth of the
agent

 Analytic epidemiology
 built around the analysis of the relationship
between two items
 Exposures
 Effects (disease)
 looking for determinants or possible causes of
disease
 useful for
 hypothesis testing

 John Snow, physician(1813-1858)
 Outbreaks of Cholera were common in London
during the 19th century
 But what was causing the cholera? The popular
theory at the time was that bad gases caused it
(‘miasma’ theory)
Example

•Analysis by place: he
mapped the cases – most
were near Broad Street
Anecdote: People had
complained that the
water smelt bad. Cases
from further afield had
water delivered by cart
from Broad Street.
What did he do?

•Recorded deaths by
water supplier
Conclusion: Risk of
infection is highest in
people using water
Southwark and Vauxhall
water company .
What did he do?

He removed the handle from the Broad Street pump and the
number of infections fell.
Public health action

The early days…John Snow
and the Broad Street Pump

The early days…John Snow
and the Broad Street Pub

Example 2
 In the early 19th century, childbed fever was a major cause of
death among women shortly after childbirth, with mortality
rates from childbed fever as high as 25%.
 Many theories of the cause of childbed fever were popular at
the time, including atmospheric toxins, “epidemic
constitutions” of some women, putrid air, or solar and
magnetic influences.

Example 2
 Semmelweis was placed in charge of the First
Obstetrical Clinic (Vienna)
 At that time there were two obstetrical clinics, the
First and the Second.
 Pregnant women were admitted for childbirth to the
First Clinic or to the Second Clinic on an alternating
24-hour basis.
 The First Clinic was staffed by physicians and
medical students and the Second Clinic by
midwives.

Men Women Children Total
1st class 67% 3% 0 38%
2nd class 92% 14% 0 59%
3rd class 84% 54% 66% 62%
Total 82% 26% 48% 62%
EVERY HEALTH OUTCOME HAS SOME INTERESTING AND
USEFUL EPIDEMIOLOGIC CHARACTERISTIC
Death rates by social class from a certain cause
among 1,316 people
WHAT CAUSE OF DEATH IS THIS?

The previous slide shows death rates
by class of ticket on the Titanic, a
large ocean liner that sank after
colliding with an iceberg in 1912

Uses of Analytical Epidemiology
 To study the cause (or etiology) of disease(s), or
conditions, disorders, disabilities, etc.
 determine the primary agent responsible or ascertain
causative factors
 determine the characteristics of the agent or
causative factors
 define the mode of transmission
 determine contributing factors
 identify and determine geographic patterns

 Epidemiology is a core part of public health and
medical research.
 It allows the distribution of health and ill-health in a
population to be described, and possible causal
factors to be identified.
Summary

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Epidemiology lecture Epidemiology lecture

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