present: disease past: exposure
DESCRIPTION
CHP400: Community Health Program - lI. STUDY DESIGNS. Present: Disease Past: Exposure. Observational / Analytical Studies. Time is Key. Research Methodology. Present: Disease & Exposure. Present: Exposure Future: Disease. nbmmb. Cross - section. Cohort. Cohort Study. . - PowerPoint PPT PresentationTRANSCRIPT
CHP400:Community Health Program - lI
Mohamed M. B. Alnoor
Research Methodology
STUDY DESIGNSObservational / Analytical Studies
CohortStudy Present:
Disease Past: Exposure
Cross - section Cohort
Case - control
Present:Disease & Exposure
Present: Exposure Future: Disease
nbmmb
Time is Key
Content: Definition Characteristics Types & Design Advantages & disadvantages Elements
• Selection of study subjects• Obtaining data on exposure • Selection of comparison group• Follow up• Analysis
STUDY DESIGNS
ExperimentalObservational
Animal
Experiment
HumanInterventio
nClinical trial
Analytical
Case control
Descriptive
Case report
Case series
Cross section
Ecological
Cohort
Research Methodology
Cohort
Study
A cohort is a group of people who share a common characteristic or experience
Cohort study: An observational, longitudinal, analytic epidemiologic study in which a particular outcome is compared in groups of people who differ by EXPOSURE .
STUDY DESIGNS
Research Methodology
Observational / Analytical StudiesCohort Study
Definition:
Characteristics : A “cohort” is a group of people, referred to as
“disease-free population” or “population at risk”
A survey is first carried out to exclude prevalent cases from the cohort
A period of "follow-up“ is specified, for possible new cases' occurrence
We know the exposure status, looking for the disease status
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Population
People without disease
Exposed
Not expose
d
DiseaseNo
disease
Disease
No disease
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Design :
YES
NO ?
?
TIMEOUTCOME/DISEASEEXPOSURE
COHORT STUDIES(CLINICAL TRIALS)
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
YES
NO
TIMEOUTCOME/DISEASEEXPOSURE
YES
NO ?
?
CASE-CONTROL STUDIES ?
?
COHORT STUDIESCLINICAL TRIALS
TIMEOUTCOME/DISEASEEXPOSURE
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Advantages: No temporal ambiguity (suggests cause-
effect relationship) Calculation of incidence rates Factors associated with selection cannot
influence disease status and hence the results.
Several outcomes can be studied, after follow-up starts.
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Disadvantages (of prospective): Expensive
Time-consuming May be impractical Loss to follow-up may affect sample-
size
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Elements of cohort study Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Selection of study subjects General population
Whole population in an areaA representative sample
Special Exposure groups of populationOccupation/professional groups
• e.g. Radiologists exposure to X-ray
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Personal interviews / mailed questionnaire
Reviews of records
Dose of drug, radiation, type of surgery etc
Medical examination or special test
Blood pressure, serum cholesterol
Obtaining data on exposure
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Environmental survey
By obtaining the data of exposure we can classify cohorts as
Exposed and non exposed
By degree of exposure we can sub classify cohorts
Obtaining data on exposure
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
External comparisonMore than one cohort in the study for the
purpose of comparisone.g. Cohort of radiologist compared with
ophthalmologists
Selection of comparison group
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Comparison with general population ratesIf no comparison group is available we can
compare the rates of study cohort with general population.
e.g. Cancer rate of uranium miners with cancer in general population
Selection of comparison group
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
To obtain data about outcome to be determined (morbidity or death)
Mailed questionnaire, telephone calls, personal interviews
Periodic medical examinationReviewing recordsSurveillance of death recordsVideoconference, neighbors, friends and
relatives.
Follow-up
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Some loss to follow up is inevitable due to death, change of address, migration, change of occupation.
Loss to follow-up is one of the draw-backs of the cohort study.
Follow-up
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Follow up is the most critical part of the study
Attrition Reduction : Obtaining an informed consent.
Recording commitment to continue and cooperate in the study.
Tracing LOST subjects. Considering Information of lost subjects at
the time of analysis
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Keeping non-response at a low level to improve the validity.
Follow-up: Problems
Calculation of incidence rates among exposed and non exposed groups
Estimation of risk
ANALYSIS
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
dc
ba
Yes NoDisease Status
Yes
No
Exposure Status
a+b
b+d a+c
TotalStudy cohortComparison cohort c+d
NTotal
Incidence among exposed =a
a+b
Incidence among unexposed =c
c+d
ANALYSIS
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Incidence rates
Estimation of riskANALYSIS
Relative Risk (RR)
incidence of disease among exposedIncidence of disease among non-exposedRR = _____________________________
a/a+b
c/c+d_________RR =
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Estimation of riskANALYSIS
Incidence of disease among
exposed
Incidence of disease among non exposed
(a/a+b) – (c/c+d)
a/a+b
AR
AR
=
=
Incidence of disease among exposed
_______________________________
_______________
Attributable Risk (AR):
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Smoking Lung cancer Total
Smoking YES NO Total
YES 70 6930 7000
NO 3 2997 3000
Total 73 9927 10000
Calculate RR and AR for above data
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Incidence of lung cancer among smokers = 70/7000 = 10 per 1000
Incidence of lung cancer among non-smokers = 3/3000 = 1 per 1000
RR = 10 / 1 = 10
(lung cancer is 10 times more common among smokers than non smokers)
AR = (10 – 1) / 10 X 100 = 90 %
(90% of the cases of lung cancer among smokers are attributed to their habit of smoking)
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
STUDY DESIGNS
Research Methodology
Observational / Analytical Studies
Cohort Study
Exposure status is known, looking for the disease status
Calculation of incidence rates Direct estimation of risk Expensive & time-consuming Loss to follow-up is one of the main
draw-backs .