Lecture of epidemiology

Amany R.Abo-El-Seoud
Prof. of Community Medicine
Zagazig University, EGYPT

 Define: epidemiology, its uses.
 Recognize the different types of
epidemiological research methods.
 Explain how to design scientific researches by
the different methods
 Compare the different research methods
(advantages and disadvantages)

 1. Describe the health status of the population in
certain community and subsequently diagnose its
health problems.
 2. Discover the causes of diseases and
determinants of ill or good health.
 3. Discover the risk factors that predispose to
diseases of unknown etiology.
 4. Complete the clinical picture of diseases from
beginning of pathological changes till cure or
occurrence of complications.
 5. Evaluate the effectiveness of health services or
test the validity of diagnostic tests or evaluate
the prognosis

◦ Disease does not occur randomly
◦ Disease has identifiable causes which can be altered
and therefore can be prevented from developing
Health is a state of equilibrium between:
Agent Host

Environment

Define:
epidemiology,
what are its uses

 Study of epidemics & epidemic disease
(Stedman’s medical dictionary)
• The science making the obvious obscure
(epidemiologist)
• The worst taught in medical school
(medical student)

The word epidemiology comes from the Greek words
epi, meaning “on or upon,” demos, meaning “people,”
and logos, meaning “the study of.”
“Epidemiology is the study of the distribution and
determinants of health-related states or events in
specified populations, and the application of this
study to control of health problems.

 describe the natural history of diseases
 explore disease causality
- direct: biological mechanisms of disease
- indirect: social and environmental causes of
disease
 provide disease surveillance
- essential for evaluating community health
problems and setting disease control priorities
 evaluate diagnostic testing
- evaluate validity, sensitivity, specificity
- to set cutoff points for abnormalities
 evaluate prognosis
- by identifying prognostic factors

Recognize the different
types of
epidemiological
research methods.

Quantitative
studies
Qualitative
studies
- Observational
-Experimental
- Phenomenology
(phenomena)
– Ethnography (cultures,
people)

 Development of concepts which help us to
understand social phenomena in natural
settings, giving due emphasis to the meanings,
experiences, and views of all the participants
 Explaining complex phenomena not amenable
to quantitative research
Method : Focus group, observation, interview
Application (examples)
 Doctor-patient relationship, treatment
compliance, clinical decision making process,
issues on health service organization
and policy issues

Two main categories:
1. Observational studies (researcher
not intervene) : cross-sectional(descriptive)–
retrospective (case control) – prospective
(cohort)
1. Experimental studies (researcher intervene)
exposure status is assigned
(intervention)

Cross-
sectional
Case -
series
Retrospective
Case control
Prospectiv
e
Cohort
Follow up
history Time now Future

Descriptive
Cross-
sectional
Case
series/study
Analytical
Case-
control
Cohort

 Descriptive Epidemiology deals with the
questions: Who, What, When, and Where?
 Analytic Epidemiology deals with the
remaining questions: Why and How?

 Study the prevalence of diabetes in Zagazig
 Study the relation between smoking and lung
cancer
 Study women satisfaction for antenatal care
 Study the effect of obesity on fetal outcome
 Study the risk of iodine deficiency on mental
retardation
 Study suggestions for improvement of
in zagazig hospital

Explain how to design
scientific researches
by the different
methods

Specified
population
Health related
problem
Distribution
Risk factors

Frequency Pattern
Health events in
population
Numbers &
rates
Prevalence
Incidence
Time, person, place
When , who, where

RISK FACTORS
Social
Cultural
BehavioralBiological
Physical

Descriptive epidemiological study of any disease can provide
these knowledge: e.g.
Food borne epidemic, TB, parasitic infestation
Pattern =Time , person , place
Frequency = prevalence & incidence rates
May be : Determinants = risk factors
Descriptive epidemiology
what
when
who
where

1- Prevalence/Cross-sectional/ Surveys
 measures prevalence of disease or incidence
of an event
 Investigation of epidemic
2- Case report / case series

 Useful for assessing the burden of disease
within a population
 Valuable for planning & follow up
 Prevalence rate= number of cases (old & new)
Number examined in the same time & place

no. of new cases of disease over
a specific period of time
Incidence = -----------------------------
no. of persons at risk of disease
over that specific period of time

= prevalent cases = incident cases = deaths or recoveries

 Prevalence is a function of the incidence of
disease and the duration of disease

Prevalence & incidence
calculation.

Case report
unit of study: single person with a disease
limitation: based on experience of a single
person
provides first clues in the identification of a
disease or adverse effects of exposure
(halothane induced hepatitis)

Case series
 unit of study: group of persons with a
similar disease
 Uses:
o formulation of criteria for diagnosis
o formulation of indications for treatment
o identification of prognostic factors
o determination of survival rates

 a-Cheap, rapid, easy
 b-Can use large sample of the population.
 c-Assess health status, and health problems
and indicate priorities for health care
planning. Assess customer’s satisfaction for
health care.
 d-Provides the base-line data for further
studies if the problem is not studied before.
 e- Cross sectional study is the most
convenient first step in the investigation of
the cause of the outbreak or epidemic.

Sequence of events cannot be ascertained
 Not useful for diseases of short
period of time
Not practical in studying rare
diseases
Case reports are susceptible to bias
Case reports can not be used to make
treatment decisions.

 Epidemiology and laboratory science converge
to provide the evidence needed to establish
causation. A team of epidemiologists were able
to identify a variety of risk factors during an
outbreak of a pneumonia or cholera epidemic
etc..
Epidemiology Laboratory
Evidence of causation

 Two groups: one is the case group “diseased’ and
the other is the control group ‘healthy".
 - Both diseased group and the control group
must be matched in everything except the
disease i.e matched in age, sex, socioeconomic
class, occupation, residence etc.
 -The collection of data is retrospectively i.e we
ask both cases and controls about their past
exposure of the risk factor under study.
 -The proportion of those exposed to a certain
factor in each group is compared.

Requirements for valid results
Cases must be representative of all those
with disease and clearly defined.
Controls must be representative of all those
without the disease and come from same
community or source as the cases.
There must be a control ( 1 or more) for
each case

Had Exposure No Exposure
Study
Population
Cases Controls
No ExposureHad Exposure

Used to find a relation between disease and
exposure
Outcome (disease)
Exposure
(risk)
diseased healthy Total
Yes a b a + b
No c d c + d
Total a + c b + d a + b + c + d

Odd’s Ratio (OR)
proportion of those with history of exposure to
the factor among the cases (a/a+c) is compared
to those with history of exposure (b/b+d) to the
factor among the controls
OR = ad/bc

Autism
MMR
Vaccine? Yes No
Total
Yes 130 115 245
No 120 135 255
Total 250 250 500
a x d 130 x 135
OR = = = 1.27
b x c 115 x 120

Disease Risk Odds Ratio 95% CI p-value
Cancer lung Smoking 2.4 1.3 – 4.4 0.004
Cancer breast Trauma 1.9 1.3 – 2.8 0.001
Infarction Obesity 1.3 1.0 – 1.7 0.04
Duodenal ulcer HP 3.7 1.0 – 5.7 0.04
Diabetes m. Viral infection 0.9 0.5 – 1.8 0.80
Cancer cervix Genital warts 0.4 0.2 – 1.0 0.05

advantages
 Good for unusual or
rare diseases
 Quick, easy, cost-
effective
 Can use secondary
data on disease
 More easily replicated
 Suitable for more than
one risk at a time
 Can test hypotheses
disadvantages
 Uncertainty is
exposure-disease time
relationship
 Representativeness of
cases or controls
 Memory problems
 Rare exposure
 Survivor problem
 Bias potential
(selection)

Which of these research questions can be
answered by case-control study? how?
1- Evaluation of performance in a health unit
2-Relation between vitamin B deficiency
&nerve conduction
3- Relation between X (rare disease) and Y risk
factor
4- Relation between cancer skin and Y rare
exposure risk

 Design case-control study to find the relation
between contraceptive pills and deep venous
thrombosis.
 Calculate odds ratio for this relationship:
Liver cirrhosis No cirrhosis
Bilharziasis 40 20
No bilharziasis 10 80

groups of subjects are chosen on the basis
of having been exposed to a factor or not
groups are followed up to identify those
who develop the disease or outcome

Uses
to test prognostic factors
to directly measure risk of development of
disease or outcome
provide more definitive information about
disease etiology
preferred for study of rare exposures

Disease No Disease
Healthy
Population
Exposed Non-exposed
No DiseaseDisease
Exposure is
self selected
Follow through
time

Analysis: Assess the strength of an association
between an exposure and the outcome of
interest
Relative Risk or Risk Ratio(RR)
 proportion of subjects with the disease or
outcome among the exposed (a/a+b) is
compared to proportion of subjects with the
disease or outcome among the unexposed
(c/c+d)
RR = a/a+b ÷ c/c+d

Attributable Risk (AR)
estimate of the amount of risk that is
attributable to the risk factor
AR = a/(a+b) - c/(c+d)

Escherichia coli
hamburger
Yes No
Total
Yes 23 10 33
No 7 60 67
Total 30 70 100
a / (a + b) 23 / 33
RR = = = 6.67
c / (c + d) 7 / 67

Advantages
provides direct estimate of risk
Time sequence can be ascertained
less biases of recall and observation
controls easier to assemble
variations in exposure can be followed-
up
unsuspected effects of the exposure
may be observed

Disadvantages
more expensive
follow-up period may be long
high drop out rate
large sample size required
change in exposure rates over long periods
of time

Quantitative
studies
Qualitative
studies
- Observational
-Experimental
- Phenomenology (phenomena)
– Ethnography (cultures, people)

Two main categories:
1. Observational studies (researcher not
intervene) : cross-sectional(descriptive)–
retrospective (case control) – prospective
(cohort)
2. Experimental studies (researcher intervene)
exposure status is assigned
(intervention)

Three different ways of classifying intervention
studies
I. Based on population studies
◦ Clinical trial: on patients in clinical settings
◦ Field trial: on healthy people in the field
◦ Community trial: on the community as a whole
II. Based on design
◦ Uncontrolled trial: no control (self-control/cross-
over)
◦ Non-randomized controlled: allocation not
random
◦ Randomized control: Allocation random

Randomization: random allocation of study
subjects
into treatment & control groups
 Outcome/endpoint
Improvement ( desired effect) and side effects
Must be exactly defined
Measured comparably in all study groups
 Blinding: Denying information on
treatment/control status.
Single – subjects (placebo effect)
Double - subjects & investigators
Triple blind - subjects & investigators & statisticians

New treatment Current treatment/
no treatment
Improved Not
improved
Improved Not
improved
Intervention
Outcome
Selection of subjects
-similar features
-inclusion/exclusion criteria
Allocation of subjects
Data collection
Masking (blinding)
Define population

 Evaluate new forms of therapy and prevention
Treatment or drugs
Health care technology - device
Methods of primary prevention - screening
Organizing and delivering health services (community
trial)
Impact of new policies in health care and health care
financing (community trial)

Advantages:
 Strongest design - randomization, minimal
selection bias
 Must be ethical - no harmful intervention,
no poor clinical outcome

Disadvantages:
 Difficult for intervention in rare disease / rare
outcome
 Participation of subjects is crucial
 Non-compliance (people who complaint are
very different from those who are not)
* Drop-out: not adherence to experimental
regimen, loss to follow-up
* Drop-in: not adherence to control regimen
 Compliance -Need monitoring
 Most costly

Surgical Medical
randomized
randomized
Refuse surgery Require surgery
No surgerySurgery
Most common
Parallel
A
B
Cross-over
• Planned – wash out
• Unplanned

Source: Grimes DA, Schulz KF. Lancet 2002; 359: 58

1. Research questions
2. Occurrence of disease / exposure -
rare/ common
3. Ethical issue
4. Resources- money, manpower, machine
Several designs may well be suitable for a
particular study. Choice must then be guided by
considerations of strength of design, cost and
ethics.

Study design Treatment Diagnosis Prognosis Agreement Classification
RCT ## - - - -
COHORT # - ## - ##
CASE-CONTROL - - - - -
CROSS SECTIONAL - ## - ## ##
CASE SERIES # # # - #
Design of research depends on research Q
Treatment efficacy/safety = RCT
Diagnostic methods, prevalence = cross- section
Prognosis- outcome = cohort
satisfaction measurement = cross- sectional
relation between risk and disease = case-
control, cohort

1. Randomized controlled trial
2. Cohort study
3. Case control study
4. Cross sectional study
5. Case report or case series
6. Experimental animals
7. Qualitative research

 Design a research to find out the relation
between obesity and diabetes (use 2 different
methods)
 Design a research to find out which is better
drug A or drug B for hypertension

 Advantages
 Disadvantages
 Requirements
 Validity
Gold standard test
test diseased healthy Total
positive a b a + b
negative c d c + d
Total a + c b + d a + b + c + d

Lecture of epidemiology

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