epidemiological study designs
TRANSCRIPT
The 6 D’s of Health Outcomes Death
Disease
Discomfort
Disability
Dissatisfaction
Destitution
A bad outcome if Untimely
A set of Symptoms, Signs and Lab results
Symptoms such as pain, nausea and itching
Impaired ability to go about usual activities
Emotional reaction to disease and its care
Financial Cost of Illness
Clinical QuestionsAbnormality Is the patient sick or wellFrequency How often does disease occurDiagnosis How accurate are the diagnostic testsCause What conditions lead to diseaseRisk What factors are associated with the
diseasePrognosis What are the consequences of diseaseTreatment How does treatment change the coursePrevention Does an intervention prevent disease
Does an early detection improve the course
Cost How much will care cost
Non Interventional StudiesNon Interventional Studies(Observational)
Interventional StudiesInterventional Studies
(Experimental)(Experimental)
Epidemiological StudiesDescriptive Studies
Describes Disease occurrence in a population Incidence Prevalence Survivial
Analytic StudiesTo determine etiology of disease
Cohort Case Control Cross Sectional
EPIDEMIOLOICAL METHODS
OBSERVATIONAL STUDIES
(NON INTERVENTIONAL)
EXPERIMENTAL STUDIES
(INTERVENTIONAL)
Descriptive studies
AnalyticalStudies
Cross Sectional Case Control
Cohort
Randomized Controlled Trials
Field trials
Community TrialsLongitudnal
Descriptive•Case report•Case series•Survey
Analytic
Observational•Cross sectional•Case-control•Cohort studies
Experimental•Randomized controlled trials
Strength of evidence for causality between a risk factor and outcome
Epidemiological sequence Observation – data collection
Counting cases and events
Relating cases and events to population at risk
Making comparison
Developing hypothesis
Testing of hypothesis by analytical studies
Making scientific inferences
Conducting experimental studies
Interventions/Evaluation
1. DESCRIPTIVE STUDY
FIRST THING’S FIRST…..!!!!
Definition
It involves the systematic collection and presentation of data to give a clear picture of a particular situation.
Descriptive can be carried out on small or large scales in community
ConcernsDescriptive study is concerned with the following questions:a. When is the disease occurring (Time)b. Where it is occurring (Place)c. Who is getting the disease (Person)
T.P.P
UsesProvides data regarding
The magnitude of the disease load
The types of disease problem in the community in term of morbidity and mortality rate and ratio
Provides “clues” to the disease etiology
Helps in Formulation of etiological hypothesis
Helps in planning, implementation and evaluation of health services/programmes
ProceduresDefining the population to be studiedDefining the disease under studyDescribing the disease by
Time Place Person
Measurement of the disease Cross Sectional Study Logitudinal Study
Comparing with known indicesFormulation of an etiological hypothesis
Measurement of Diseasea. Cross Sectional Study
b. Longitudinal Study
Cross sectional studies
Based on a single examination of a cross section of population at one point in time, results of which can be projected on the whole population provided the sampling has been done correctly.
Uses
a. More useful in chronic diseases
b. To find more about disease rather than its etiology
CharacteristicsPhysical characteristics of people, material
and environment
Socio-economic characteristics e.g., age, education , marital status, number of children and income
Behavior of people like knowledge, attitude and beliefs (KAP)
Events that occur in population
Advantages of CSSMay study several outcomes
Control over selection of subjects
Control over measurements
Relatively short duration
First step for cohort study
Yields prevalence
Disadvantages of CSSDoes not establish cause/effect ratio
Potential bias in measuring exposure
Potential survival bias
Not feasible for rare disease
Does not yield incidence
Longitudinal Study
Based on multiple observations in the same population over a prolong period of time.
Uses of Longitudinal Study
Natural History of Disease
Identifying Risk factors
Finding out incidence rate
Cross Sectional Vs Longitudinal
COMPARATIVE or ANALYTICAL STUDY
An ANALYTICAL STUDY attempts to establish causes or risk factors for certain problems. This is done by comparing two or more groups, some of which have or develop the problem and some of which have not.
Analytic EpidemiologySecond major type of epidemiological studies
Subject of interest is individual within the population
The objective is to test hypothesis
The study determines whether or not a statistical association exists between a disease and suspected factor
Strength of association, if it exists
CASE CONTROL STUDY
In a CASE-CONTROL STUDY, the investigator compares one group among whom a problem is (e.g., malnutrition) with another group, called a control or comparison group, where the problem is absent to find out what factors have contributed to the problem.
CASE CONTROL STUDY
Often called retrospective study
First approach to test causal hypothesis
PropertiesBoth exposed and outcome (Disease) have
occurred before the start of study
The study proceeds backward from effect to cause
It uses a control or comparison group to support or refute an inference
MethodSelection of cases and Controls
Matching
Measurement of Exposure
Analysis and Interpretation
Exposure Rates
Estimation of Risk (Relative Risk & Odds Ratio)
A
C D
B
Cases
Exposure -
Exposure +
Controls
A+C B+D
2 x 2 Contingency Table for Cases and Controls
Total Cases
Exposure Among Cases
A/(A+C)
Exposure Among Controls
B/(B+D)
Advantages of case control studyRelatively easy to carry outRapid and inexpensiveParticularly suitable to investigate rare
diseasesNo risk to subjectReveals the study of several different
etiological factorsRisk factors can be identifiedNo follow up in the futureMinimum ethical problems
Disadvantages of case control study
Problems of bias
Selection of appropriate case control group may be difficult
Cannot measure incidence, only relative risk is measured
Randomized controlled trialsInvestigator controls the predictor variable
(intervention or treatment)Major advantage over observational studies
is ability to demonstrate causalityRandomization controls unmeasured
confoundingOnly for mature research questions
PopulationPopulation
SampleSample
TreatmentTreatment Dx No Dx
ControlControl Dx No DxPlaceboPlacebo
RandomizationRandomization
Steps in a randomized controlled trial
1. Select participants high-risk for outcome (high incidence) Likely to benefit and not be harmed Likely to adhere
2. Measure baseline variables3. Randomize
Eliminates baseline confounding Types (simple, stratified, block)
Analysis of randomized controlled trial
Analyzed like cohort study with RRIntention to treat analysis
Most conservative interpretationInclude all persons assigned to intervention
group (including those who did not get treatment or dropped out)
Subgroup analysisGroups identified pre-randomization
Steps in a randomized controlled trial
4. Blinding the intervention As important as randomization Eliminates
co intervention biased outcome ascertainment biased measurement of outcome
5. Follow subjects Adherence to protocol Lost to follow up
6. Measure outcome Clinically important measures Adverse events
What is Blinding?What is Blinding?Single blind - participants are not aware
of treatment groupDouble blind - both participants and
investigators unawareTriple blind - various meanings
persons who perform tests outcome adjudicators safety monitoring group
Single blind - participants are not aware of treatment group
Double blind - both participants and investigators unaware
Triple blind - various meanings persons who perform tests outcome adjudicators safety monitoring group
Why blind?: Co interventions Unintended effective interventions
participants use other therapy or change behavior study staff, medical providers, family or friends
treat participants differently
Nondifferential - decreases power Differential - causes bias
Why blind?: Biased Outcome Ascertainment or adjudicationWhy blind?: Biased Outcome
Ascertainment or adjudication If group assignment is known
participants may report symptoms or outcomes differently
physicians or investigators may elicit symptoms or outcomes differently
Study staff or adjudicators may classify similar events differently in treatment groups
Problematic with “soft” outcomes investigator judgement participant reported symptoms, scales
If group assignment is known participants may report symptoms or outcomes
differently physicians or investigators may elicit symptoms
or outcomes differently Study staff or adjudicators may classify similar
events differently in treatment groups
Problematic with “soft” outcomes investigator judgement participant reported symptoms, scales
High Quality Randomized TrialsTamper-proof randomizationBlinding of participants, study staff, lab staff, outcome ascertainment and adjudication
Adherence to study intervention and protocol
Complete follow-up
COHORT STUDYIn a COHORT STUDY, a group of individuals
that is exposed to a risk factor (study group) is
compared with a group of individuals not
exposed to the risk factor (control group).
The researcher follows both groups over time
Compares the occurrence of the problem
related to the risk factor in the two groups
Determines whether a greater proportion of
those with the risk factor are indeed affected
CONCEPT OF COHORT
The term Cohort is defined as group of people who share a common characteristic or experience within a defined time period e.g., age, occupation, exposure of a drug or vaccine, birth cohort and marriage cohort etc.
Distinct features of cohort studyThe cohort are identified prior to the
appearance of disease under study.The study groups so defined observed over a
period of time to determine the frequency of the disease among them
The study proceeds forward from cost to effect
A
C D
B
Disease +
Risk Factor -
Risk Factor +
Disease -
A+B
C+D
2 x 2 Contingency Table for Cohort Study
Total
Exposed
Incidence Among Exposed
A/(A+B)
Incidence Among Unexposed
B/(B+D)
Elements of a cohort studySelection of the study groupObtaining data on exposureSelection of the comparison groupFollow upAnalysis
Strengths of cohort studiesKnow that predictor variable was present before
outcome variable occurred (some evidence of causality)
Directly measure incidence of a disease outcome
Can study multiple outcomes of a single exposure (RR is measure of association)
Weaknesses of cohort studiesExpensive and inefficient for studying rare
outcomesHERS vs. WHI
Often need long follow-up period or a very large populationCARDIA
Loss to follow-up can affect validity of findingsFramingham
Selection of the study groupGeneral populationSelected group of population e.g.; doctors,
teachers, nurses, school children etc.
Exposure DataPersonal interviewMailed
questionnaireReview of recordsMedical
examinationEnvironmental
survey
Follow upPeriodic examination of each member of the
cohortReviewing physician and hospital recordRoutine surveillance of death recordsMailed questionnaireTelephone callsPeriodic home visit (on annual basis)
Follow up can be done through
• Online
Interviews may be conducted
Face-to-face
• Over the Telephone
Analysis
Relative Risk
Attributable Risk
Odds ratio
Relative Risk (RR)
Ratio of incidence of the disease (or
death) among exposed and the incidence
among non-exposed.
It is a direct measure (or index) of the
“strength” of the association between
suspected cause and effect
Odds Ratio (OR) Measure of the strength of the association between
risk factor and outcome.
The derivation of the Odds Ratio is based on three assumptions:
- the disease being investigated must be relatively rare
- the cases must be representative of those with the disease
- the controls must be representative of those without disease
Analysis
Relative risk (RR)Ie
RR = -----Io
Attributable risk (AR)
Ie-IoAR = -------- X100
Ie
• WhereasIe : Incidence among
exposedIo : Incidence among non
exposed
Difference between BIAS and CHANCE
BIASIt is deviation of results, or inferences from the truth or processes leading to such deviation. It is a systematic error
CHANCEIt is a random error and may account for an apparent association and make it appear real when it is not (Type I or alpha error). It may lead to an association being overlooked or missed when it truly exists (Type II or Beta error)
Comparison Case control
Retrospective Hospital based Quick Easy to conduct Small sample size Less expensive Rare diseases None
Cohort Prospective/
longitudinal Community based Time consuming Logistically difficult Large sample size Very expensive Common disease Incidence
Case Control VS Cohort Study
FactorsPresent
Absent
Risk FactorsExposed
Unexposed
DiseasePresent (Cases)
Absent (Controls)
DiseasePresent on Followup
Absent on Followup
CASE CONTROL STUDY
COHORT STUDY
TIME LINE
INTERVENTIONAL STUDIES
INTERVENTIONAL STUDIESIn Intervention Studies the researcher
manipulates the situation and measures the effects of this manipulation.
Usually (But not Always) 2 Groups are compared, one in which the Intervention takes place and the other group that remains “Untouched”
INTERVENTIONAL STUDIESThe 2 categories of Intervention Studies are:
1. Experimental Studies
2. Quasi-Experimental Studies
Experimental StudiesIndividuals are randomly allocated to atleast 2 groups. One group is subject to Intervention or Experiment while the other group is not. Then the outcome of the intervention is obtained by comparing the 2 groups
Diagram of Experimental Study
Study Group(Experimental)
1st Data Collection(Before Intervention)
Intervention
Last Data Collection(After Intervention)
Study Population (Sampling)
Sample Population (Randomization)
COMPARE
Control Group(Comparison)
1st Data Collection(Same Time)
No Intervention
Last Data Collection(Same Time)
Quasi-Experimental Studies
In this at least one characteristic of a true experiment is missing. This may be Either:
Missing of RandomizationMissing of separate Control group
This however always includes manipulation of independent variable that serves as intervention
Diagram of Quasi Experimental ModelStudy Group before Intervention Study Group
After
COMPARE
Control Group Before Control Group After
THANK YOU
What is IncidenceNo. of new cases of a disease or
health related event in a given population in a given time
___________________________
Total Population at risk in a given time
X 1000
What is PrevalenceNo. of new as well as Old cases of a
disease or health related event in a given population in a given time
___________________________
Total Population at risk in a given time
X 1000
What does Incidence SignifiesIt shows the RATE at which new diseases or
health problems occur in a population.
Prevalence shows the proportion of a population at risk which is affected by a disease at a specific point in time.
It is further of 2 typesPoint PrevalencePeriod prevalence.
Case 1
Case 2
Case 3
Case 4
Case 5
Case 9
Case 8
Case 6
Case 7
Case 10
1Jan,2004 28 Dec,2004
Total No of Patients admitted during this 1 year = 100
What is the prevalence of Hepatitis B on 1st January 2004.
What is prevalence of Hepatitis B during the year 2004.
Relation between Incidence and PrevalencePrevalence = Incidence x Duration of the
Disease
PREVALENCE
INCIDENCE
RECOVERY DEATH
Variations in Incidence and PrevalenceSince Incidence depends on the occurrence
of new cases of a disease, a DECREASE in Incidence may be due toEnhanced Resistance to the diseaseA change in Disease EtiologyAn effective prevention program that reduces
exposure to a known risk factor for the disease.
A DECREASE in Prevalence may be due toA decrease in IncidenceA shorter duration of the disease due to either
improved treatment methods leading to more rapid recovery or an increase in virulence leading to more rapid death.
Risk Factor and CausalityWhat is a Risk Factor
A condition, physical characteristic, or behaviour that increases the probability that a currently healthy individual will develop a particular disease.
A Risk Factor may be a causal factor of the disease in question or merely a marker for the increased probability of disease.
E.g while poor antenatal acre and drug use constitute causal factors for neonatal mortality, socioeconomic status would be considered a marker for neonatal mortality.
Risk AssessmentA number of epidemiological research
designs are used to evaluate the association between a disease and a suspected risk factor.
Types of Epidemiological StudiesDescriptive Studies
Also termed as Cross-sectional studies they determine the disease frequency or prevalence of a condition. Surveys are one example
Analytic StudiesObservational StudiesExperimental Studies
Analytical StudiesObservational
Case Control StudiesCohort Studies
Prospective Cohort Studies Retrospective Cohort Studies.
Case control studies: Case-control studies are those in which persons with a specified condition (the cases) and pesons without the condition (the controls) are selected for study. The proportion of cases and controls with certain characteristics or exposure is then measured and compared. For example, knowing that there are 10 school children with purple spots in grade 3, a set of other third grade children from the same school but without purple spots would be identified as controls, and analysis done to see what different exposures the purple-spotted children had than the non-spotted
Cohort studies: groups of individuals with some common feature (age and geography, for example) are identified for study over time to learn about differing health and illness experiences. For example, one might enroll in a study all third graders in a school and follow them until graduation, attempting to identify the differences in experiences of those who maintained a body weight close to recommended and those who did not.
EXPERIMENTAL STUDIESQuasi-Experimental StudiesTrue Experimental Studies
RandomizationControl Group
ExcerciseIn 1998 an outbreak of Cholera in a
Peshawar Suburb shows the following Data.
Cases presenting Cases presenting with loose motions, with loose motions, vomiting of acute vomiting of acute duration in the last 48 duration in the last 48 hourshours
People in the same People in the same area who didn’t have area who didn’t have gastrointestinal gastrointestinal symptomssymptoms
Using community Using community water supplywater supply 135135 5555
Using own dugged Using own dugged wellswells 22 103103
From 1960 to 1992, 35250 adults aged 20-25 years were followed up for habits of smoking and were assessed for presence of Bronchogenic Carcinoma with following results.
CT Scan evidence of CT Scan evidence of Ca LungCa Lung
No evidence of ca No evidence of ca LungLung
People who started People who started and continued with and continued with SmokingSmoking
435435 2016520165
People who didn’t People who didn’t SmokeSmoke 1010 1464014640