lecture of epidemiology
TRANSCRIPT
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Amany R.Abo-El-SeoudProf. of Community MedicineZagazig University, EGYPT
RESEARCH METHODOLOGY(Epidemiological methods)
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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)
The intended learning objectives are:
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Why do we do medical research?
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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
Objectives of medical research:
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◦ 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
Important Assumptions:
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Define: epidemiology, what are its uses
1st objective
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Study of epidemics & epidemic disease (Stedman’s medical dictionary)• The science making the obvious obscure (epidemiologist)• The worst taught in medical school (medical student)
EPIDEMIOLOGY
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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.
EPIDEMIOLOGY
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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
Uses of Epidemiology in Clinical
Medicine
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Recognize the different types of epidemiological
research methods.
2nd Objective
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Types of epidemiological studies
Quantitative studies
Qualitative studies
- Observational -Experimental
- Phenomenology (phenomena)– Ethnography (cultures, people)
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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, interviewApplication (examples) Doctor-patient relationship, treatment
compliance, clinical decision making process, issues on health service organization
and policy issues
Qualitative study
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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)
Types of quantitative Studies
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OBSERVATIONAL STUDIES According to time of research
Cross-sectiona
lCase -series
RetrospectiveCase control
Prospective
CohortFollow up
history Time now Future
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Descriptive
Cross-sectional
Case series/study
Analytical
Case-control
Cohort
OBSERVATIONAL STUDIESin relation to research question
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Descriptive Epidemiology deals with the questions: Who, What, When, and Where?
Analytic Epidemiology deals with the remaining questions: Why and How?
Descriptive vs. Analytic Epidemiology
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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
Quiz: choose type of ES
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Explain how to design scientific researches
by the different methods
3rd Objective
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Specified population
Health relatedproblem
Distribution
Risk factors
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Frequency Pattern
Health events in population
Numbers & ratesPrevalenceIncidence
Time, person, placeWhen , who, where
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RISK FACTORS
Social
Cultural
Behavioral Biological
Physical
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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
whatwhatwhenwhen
whowhowhere
where
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Descriptive
Cross-sectional
Case series/study
Analytical
Case-control
Cohort
OBSERVATIONAL STUDIESin relation to research question
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1- Prevalence/Cross-sectional/ Surveys measures prevalence of disease or
incidence of an event Investigation of epidemic
2- Case report / case series
TYPES OF DESCRIPTIVE STUDIES
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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
Study Prevalence rate
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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
Study Incidence rate
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Prevalence and Incidence
= prevalent cases = incident cases = deaths or recoveries
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Prevalence is a function of the incidence of disease and the duration of disease
Prevalence and Incidence
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Prevalence & incidence calculation.
QUIZ
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Descriptive
Cross-sectional
Case series/study
Analytical
Case-control
Cohort
OBSERVATIONAL STUDIESin relation to research question
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Case reportunit of study: single person with a
diseaselimitation: based on experience of a
single personprovides first clues in the identification
of a disease or adverse effects of exposure
(halothane induced hepatitis)
Types of Descriptive Studies (cont.)
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Case series unit of study: group of persons with
a similar disease Uses:
o formulation of criteria for diagnosiso formulation of indications for
treatmento identification of prognostic factorso determination of survival rates
TYPES OF DESCRIPTIVE STUDIES
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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.
Advantages of cross-sectional studies
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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 biasCase reports can not be used to make treatment decisions.
Disadvantages of cross-sectional/ surveys/ Case Series studies
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Descriptive
Cross-sectional
Case series/study
Analytical
Case-control
Cohort
OBSERVATIONAL STUDIESin relation to research question
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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..
Analytical studies search for causes
Epidemiology Laboratory
Evidence of causation
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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.
ANALYTIC STUDIESHow to design Case-control Study?
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Requirements for valid resultsCases 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
CASE-CONTROL STUDIES
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Case-Control Studies
Had Exposure No Exposure
StudyPopulation
Cases Controls
No ExposureHad Exposure
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Analysis of case control studies
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
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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
Analysis of case-control studies
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Odds Ratio Example
Autism
MMR Vaccine? Yes No
Total
Yes 130 115 245
No 120 135 255
Total 250 250 500
a x d 130 x 135OR = = = 1.27
b x c 115 x 120
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How to interpret “OR”?
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
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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)
Case control studies
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QUIZWhich of these research questions can be answered by case-control study? how?1- Evaluation of performance in a health unit2-Relation between vitamin B deficiency &nerve conduction3- Relation between X (rare disease) and Y risk factor4- Relation between cancer skin and Y rare exposure risk
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Design case-control study to find the relation between contraceptive pills and deep venous thrombosis.
Calculate odds ratio for this relationship:
QUIZ
Liver cirrhosis No cirrhosis
Bilharziasis 40 20
No bilharziasis 10 80
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Descriptive
Cross-sectional
Case series/study
Analytical
Case-control
Cohort
OBSERVATIONAL STUDIESin relation to research question
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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
TYPES OF ANALYTIC STUDIES(2) Cohort Studies = follow up study
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Usesto test prognostic factorsto directly measure risk of development of disease or outcome
provide more definitive information about disease etiology
preferred for study of rare exposures
COHORT STUDIESfollow up / prospective
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Cohort Studies
Disease No Disease
HealthyPopulation
Exposed Non-exposed
No DiseaseDisease
Exposure isself selected
Follow throughtime
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Analysis: Assess the strength of an association between an exposure and the outcome of interestRelative 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
COHORT STUDIES
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Attributable Risk (AR)estimate of the amount of risk that is attributable to the risk factor
AR = a/(a+b) - c/(c+d)
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Relative Risk Example
Escherichia coli
hamburgerYes No
Total
Yes 23 10 33
No 7 60 67
Total 30 70 100
a / (a + b) 23 / 33RR = = = 6.67
c / (c + d) 7 / 67
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Advantagesprovides direct estimate of riskTime sequence can be ascertained less biases of recall and observation
controls easier to assemblevariations in exposure can be followed-up
unsuspected effects of the exposure may be observed
COHORT STUDIES
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Disadvantagesmore expensivefollow-up period may be longhigh drop out ratelarge sample size requiredchange in exposure rates over long
periods of time
COHORT STUDIES
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Types of epidemiological studies
Quantitative studies
Qualitative studies
- Observational-Experimental
- Phenomenology (phenomena)– Ethnography (cultures, people)
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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)
Types of quantitative Studies
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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
Experimentalintervention studies
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Randomization: random allocation of study subjects
into treatment & control groups Outcome/endpoint
Improvement ( desired effect) and side effectsMust be exactly defined Measured comparably in all study groups
Blinding: Denying information on treatment/control status. Single – subjects (placebo effect) Double - subjects & investigatorsTriple blind - subjects & investigators & statisticians
Experimental studies
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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 collectionMasking (blinding)
Randomized controlled trial
Define population
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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)
Randomized controlled trial
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Advantages:
Strongest design - randomization, minimal
selection bias
Must be ethical - no harmful intervention,
no poor clinical outcome
Randomized controlled trial
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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
Randomized controlled trial
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Types of randomized clinical trials
Surgical Medical
randomized
randomized
Refuse surgery Require surgery
No surgerySurgery
Most common Parallel A B
Cross-over
• Planned – wash out
• Unplanned
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Classification of Study Designs
Source: Grimes DA, Schulz KF. Lancet 2002; 359: 58
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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 aparticular study. Choice must then be guided
byconsiderations of strength of design, cost
and ethics.
Which study design?
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Study design Treatment Diagnosis Prognosis Agreement Classification
RCT ## - - - -COHORT # - ## - ##CASE-CONTROL - - - - -CROSS SECTIONAL - ## - ## ##CASE SERIES # # # - #
SummaryDesign of research depends on research QTreatment efficacy/safety = RCTDiagnostic methods, prevalence = cross- sectionPrognosis- outcome = cohortsatisfaction measurement = cross- sectionalrelation between risk and disease = case-control, cohort
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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
Types of study designs
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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
QUIZ
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Sampling techniquesAdvantages of sampling
Types of sampling
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Advantages Disadvantages Requirements Validity
Screening test
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
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