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Overview of Overview of Study DesignsStudy Designs
Study DesignsStudy Designs
Experimental
Randomized Controlled Trial
Group Randomized
Trial
Observational
Descriptive Analytical
Cross-sectional
Ecological
Cohort
Case-control
Experimental v. Experimental v. ObservationalObservational
““Gold Standard”Gold Standard” Investigator Investigator
controls controls exposureexposure
Less commonLess common
Investigator Investigator observes observes exposuresexposures
Most commonMost common
4
INTERVENTION GROUP
OUTCOMEASSESSMENT
Present Future
TIME
Experimental Study: Experimental Study: Randomized Controlled Randomized Controlled
TrialTrial
Randomly Assigns
Intervention
Comparison
Randomized Controlled Randomized Controlled TrialTrial
Randomization is random distribution Randomization is random distribution or allocation of subjects into groupsor allocation of subjects into groups Treatment or intervention groupTreatment or intervention group Comparison group (placebo, another tx, no Comparison group (placebo, another tx, no
tx, usual care)tx, usual care) Helps ensure that both groups are Helps ensure that both groups are
similar re: known and unknown factorssimilar re: known and unknown factors Two types of RCTsTwo types of RCTs
Preventive trialsPreventive trials Therapeutic trialsTherapeutic trials
Group Randomized TrialGroup Randomized Trial
Unit of assignment = groupUnit of assignment = group Unit of analysis = individual Unit of analysis = individual
(usually)(usually) Investigator controls exposure statusInvestigator controls exposure status
Randomly assigns group to intervention Randomly assigns group to intervention groupgroup
Exposure assessed Exposure assessed beforebefore outcome outcome
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Experimental StudyExperimental Study
StrengthsStrengths Able to Able to
demonstrate causal demonstrate causal associationassociation
Randomization Randomization deals with known deals with known and unknown and unknown confoundersconfounders
Investigators Investigators directly control directly control interventionintervention
LimitationsLimitations Ethical concerns Ethical concerns
(equipoise)(equipoise) Gen practice ≠ Gen practice ≠
artificial artificial ExpensiveExpensive Power often an Power often an
issueissue
Observational StudiesObservational Studies Most common, by necessityMost common, by necessity Observes nature without interventionObserves nature without intervention
Takes advantage of people’s natural Takes advantage of people’s natural exposures (choice, occupation, environment, exposures (choice, occupation, environment, residence)residence)
Collect data and statistically analyze Collect data and statistically analyze resultsresults Goal is to “mimic” experimental studyGoal is to “mimic” experimental study Statistics more important and usually more Statistics more important and usually more
complexcomplex Can be descriptive or analyticCan be descriptive or analytic
Study DesignsStudy Designs
Experimental
Randomized Controlled Trial
Group Randomized
Trial
Observational
Descriptive Analytical
Cross-sectional
Ecological
Cohort
Case-control
Cohort StudiesCohort Studies Two ways to assemble cohort:Two ways to assemble cohort:
Group of individuals with common Group of individuals with common characteristic or experience (then assess characteristic or experience (then assess exposure exposure beforebefore outcome) outcome)
Select subjects without outcome according Select subjects without outcome according to exposure status (esp important for rare to exposure status (esp important for rare exposure)exposure)
Followed over time to determine Followed over time to determine incidence of symptoms, disease, or deathincidence of symptoms, disease, or death
Analysis focuses on risk of outcome in Analysis focuses on risk of outcome in exposed compared to unexposed groupsexposed compared to unexposed groups Unexposed is referent or comparison groupUnexposed is referent or comparison group
Prospective Cohort Prospective Cohort StudiesStudies
Only include outcome-free individualsOnly include outcome-free individuals At risk of developing outcomeAt risk of developing outcome
Followed into future to observe Followed into future to observe outcomesoutcomes Can take long time to completeCan take long time to complete Important to re-assess exposures over time Important to re-assess exposures over time
Useful design for rare exposuresUseful design for rare exposures Not for rare outcomesNot for rare outcomes
Advantage: exposure assessed before Advantage: exposure assessed before subject knows outcome status; temporal subject knows outcome status; temporal sequencesequence
12
EXPOSURE ASSESSMENT
OUTCOMEASSESSMENT
Present Future
TIME
Prospective Cohort StudyProspective Cohort Study
Exposed
Unexposed
Retrospective Cohort Retrospective Cohort StudiesStudies
Both exposure and outcome occur before Both exposure and outcome occur before studystudy Studies only prior outcomesStudies only prior outcomes
Historical cohort reconstructed from Historical cohort reconstructed from existing data sources before study beginsexisting data sources before study begins
Example: Effects of pesticide exposure on Example: Effects of pesticide exposure on cancer mortality (cohort of factory workers cancer mortality (cohort of factory workers employed by manufacturer identified from employed by manufacturer identified from personnel records)personnel records)
Need good records with info on many Need good records with info on many variablesvariables
Sometimes good outcome info not availableSometimes good outcome info not available
14
EXPOSURE ASSESSMENT
OUTCOMEASSESSMENT
Past PresentTIME
Retrospective Cohort StudyRetrospective Cohort Study
Exposed
Unexposed
15
Cohort StudyCohort Study
StrengthsStrengths Easier to Easier to
differentiate cause differentiate cause from effectfrom effect
Direct estimation of Direct estimation of incidenceincidence
Able to examine Able to examine multiple outcomesmultiple outcomes
Efficient for rare Efficient for rare exposuresexposures
ProspectiveProspective Reduces recall biasReduces recall bias
LimitationsLimitations Large sample sizeLarge sample size ProspectiveProspective
Long-term follow-upLong-term follow-up ExpensiveExpensive
Case-Control StudiesCase-Control Studies Both exposure and outcome have Both exposure and outcome have
occurredoccurred Classified according to outcome status Classified according to outcome status
beforebefore exposure ascertained exposure ascertained Cases with outcome selected from well-Cases with outcome selected from well-
defined source populationdefined source population Controls without outcome* sampled Controls without outcome* sampled
from population that produced casesfrom population that produced cases Analysis focuses on odds of Analysis focuses on odds of exposureexposure
in cases compared to controlsin cases compared to controls
Case-Control StudyCase-Control Study Grouped on basis of past or current Grouped on basis of past or current
outcomeoutcome Outcome already occurred before Outcome already occurred before
exposure assessedexposure assessed May lead to biased recallMay lead to biased recall May be difficult to establish temporalityMay be difficult to establish temporality
Useful design for rare outcomesUseful design for rare outcomes Not for rare exposuresNot for rare exposures
Advantages: Advantages: Less expensive than cohort (usually)Less expensive than cohort (usually) Can look at multiple exposuresCan look at multiple exposures
18
EXPOSURE ASSESSMENT
OUTCOMEASSESSMENT
Past Present
TIME
Case-Control StudyCase-Control Study
Cases(with outcome)
Controls(without outcome)*
19
Case-Control StudyCase-Control Study
StrengthsStrengths Efficient for rare Efficient for rare
outcomesoutcomes Able to examine Able to examine
multiple exposuresmultiple exposures
LimitationsLimitations ↑ ↑ possibility of biaspossibility of bias Temporal Temporal
associationassociation Inefficient for rare Inefficient for rare
exposuresexposures
Cross-Sectional StudiesCross-Sectional Studies
Study population not selected based Study population not selected based on outcome or exposure statuson outcome or exposure status
Outcome and exposure assessed at Outcome and exposure assessed at same timesame time
Snapshot at single point in timeSnapshot at single point in time Problems inferring temporal sequenceProblems inferring temporal sequence Identify prevalent cases of long Identify prevalent cases of long
durationduration
21
EXPOSURE ASSESSMENT
OUTCOMEASSESSMENT
Past Present
TIME
Analytical Cross-Sectional Analytical Cross-Sectional StudyStudy
Exposed
Unexposed
22
Cross-Sectional StudyCross-Sectional Study
StrengthsStrengths Less time and Less time and
resources resources No follow-up time No follow-up time
involvedinvolved More representative More representative
of well-defined of well-defined general populationgeneral population
Useful when onset is Useful when onset is difficult to establish difficult to establish (incidence)(incidence)
LimitationsLimitations Temporal Temporal
associationassociation Potential for Potential for
prevalence-prevalence-incidence biasincidence bias
Not for rare Not for rare exposures or exposures or outcomesoutcomes
Reverse causalityReverse causality
23
EXPOSURE ASSESSMENT
OUTCOMEASSESSMENT
Past PresentTIME
Analytical Ecological StudyAnalytical Ecological Study
Exposed
Unexposed
Note: Often in ecological studies, the exposures and outcomes are continuous measures
24
Ecological StudyEcological Study StrengthsStrengths
Relatively quick and Relatively quick and inexpensiveinexpensive
May be only appropriate May be only appropriate design for research design for research questionquestion
May be useful when May be useful when intra-group variability in intra-group variability in exposure is small exposure is small
May provide greater May provide greater inter-group variability inter-group variability across larger ecological across larger ecological unitsunits
LimitationsLimitations Ecological fallacyEcological fallacy Secondary data Secondary data
sourcessources ConfoundingConfounding Temporal Temporal
associationassociation Migration across Migration across
groupsgroups
Characteristics of Study Characteristics of Study DesignsDesigns
Unit of observati
on
E assesse
d before outcom
e?
Investigator controls
E?
RCT Individual
Yes Yes
Group RCT
Group Yes Yes
Cohort Individual
Yes No
Case-control
Individual
No No
X-sectional
Individual
No No
Ecological
“Group” No No
•Exposure(s) of interest?•Outcome(s) of interest?•Population investigated?•Recruitment?•Study design?•Strengths?•Limitations?