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Descriptive Studies
• Record events, observations or activities,documentaries
• No comparison group or intervention• Describe an event that needs to be brought
to the attention of colleagues• May lead to additional discoveries• Exercise caution with interpretation and application of information
Observational Studies
• Include case-control, cohort (follow-up), and cross-sectional studies
• Goal is to answer questions, not simply describe a situation
• Include a comparison• Report the natural course of events• Researchers are bystanders
Subjects WithSubjects With Disease (case)Disease (case)
Subjects WithoutSubjects WithoutDisease (control)Disease (control)
Study Study PopulationPopulation
Prior Exposure to Prior Exposure to Suspected Risk FactorSuspected Risk Factor
Prior Exposure to Prior Exposure to Suspected Risk FactorSuspected Risk Factor
No Prior Exposure toNo Prior Exposure toSuspected Risk FactorSuspected Risk Factor
No Prior Exposure toNo Prior Exposure toSuspected Risk FactorSuspected Risk Factor
Present-------------Time---------------PastPresent-------------Time---------------Past
Case Control Studies
Case-control Studies:Strengths
• Efficient– Time – Money
• Useful for the study of rare diseases
• Ethical concerns are limited
Case-control Studies:Weaknesses
• Incomplete or biased records• Recall of subjects may be selective or biased• Selection of controls
– From clinic, acute care facility, or community– Who included and excluded– Matched controls
• Selection of cases• Info gathering, differences between groups,
etc.
Subjects ExposedSubjects Exposed
Subjects NotSubjects NotExposedExposed
Study Study SampleSample at Riskat Risk
Develop Measured Develop Measured OutcomeOutcome
Develop MeasuredDevelop MeasuredOutcomeOutcome
Do Not DevelopDo Not DevelopMeasured OutcomeMeasured Outcome
Do Not DevelopDo Not DevelopMeasured OutcomeMeasured Outcome
TimeTime
Cohort Studies
Cohort Studies
• Strengths– Data is collected prospectively and in a more controlled
fashion– Subjects are not required to “recall” data– More accurate assessment of “which” came first
• Weaknesses– High cost and much time required– Population selection– Loss to follow-up– Changes in subject characteristics– Surveillance bias
Case-control study: takes the outcome as the starting point of the inquiry and looks for precursors or risk factors
Cohort Study: starts with a risk factor or exposure and looks at consequences
Comparison Case Control and Cohort
Cross-sectional Studies
• Assesses exposure and outcome status simultaneously
• “Slice-in-time” design or prevalence
• Assessment made during a
single examination
Cross-sectional Studies
Subjects WithSubjects WithDiseaseDisease
Subjects FreeSubjects FreeOf DiseaseOf Disease
Study Study SampleSample
Risk Factor Risk Factor PresentPresent
Risk FactorRisk FactorPresentPresent
No Risk FactorNo Risk FactorPresentPresent
No Risk FactorNo Risk FactorPresentPresent
PRESENTPRESENT
Cross-sectional Studies:
• Evaluate a new test
• Evaluate the predictive capability of clinical features
• Identify etiological agents or causative factors
• Determine prevalence
Cross-sectional Design
• Strengths– Efficient use of time– Efficient use of money
• Weaknesses– Population selection– Subject selection– Antecedent-Consequent relationship
Experimental Studies
• Also known as a controlled trial, clinical trial, health-care trial, or intervention trial
• Goal is to answer questions, not simply describe a situation
• Include a comparison• An intervention is planned• An outcome is measured• Subjects followed forward in time
Definitions
• Retrospective Trial– Case-control study
– A study that analyzes previously collected data
– Carefully kept records are needed
• Prospective Trial:– Cohort or follow-up study
– A study in which data are collected
forward in time from the date
of study initiation
The True Experiment
• Clinical trial, controlled trial, health-care trial, intervention trial
• Tool to evaluate clinical problems• Investigator actively intervenes• Intervention applied (independent variable)• Outcome is observed (dependent variable)• Goal is to establish a causal relationship
The True Experiment
• Most effective for controlling extraneous factors affecting interpretation of results
• Limitations include:– Expense– Ethics– Time required– Complexity
The Hypothesis
• Stated in introduction section• Focuses a study• Null hypothesis
– Hypothesis of no difference or no relationship– Results held to be true until evidence shows
otherwise
• Research hypothesis: expected results of the experiment based on theory or related research
The Hypothesis
Two Tailed Hypothesis• States that an
association or difference exists
• No direction of association specified
• Statistically
stronger
One Tailed Hypothesis• States that an
association or difference exists
• Direction between the predictor & outcome variables is specified
• Statistically easier to prove
Subject Identification
• Population selection• Subject selection
– Convenience sample– Systematic sample– Random sample
• Inclusion & exclusion criteria• Author should clearly explain process
of identifying subjects
Methods of Control
• Randomization
• Placebo, active, or historical control
• Blinding
• Stratification
• Matching
• Cross-over design
Randomization
• Always implies there is a control group• Assignment procedure• Improves validity of a study• Subjects are assigned to groups using a
system independent of personal judgment or subjects themselves
• Every member of population has a predetermined probability of being chosen for a group
Randomization
• Goal is to decrease/eliminate selection bias• Individuals randomized may or may not be
representative of a larger population• Differs from random sampling (individuals
are selected for a study by chance from a larger group or population; Aim is to obtain a sample that reflects characteristics of a larger group)
Blinding
• Eliminates bias that randomization cannot eliminate
• Errors occur when patient or individual making assessment are aware of treatment administered, especially if outcome is subjective
• Important to guarantee that the observed outcome is due to the applied therapy
• Attempts to overcome placebo effect
Blinding
• Single blind: subject unaware of therapy received
• Double blind: neither the observer nor subject aware of therapy received
• Triple blind: subject, investigator, and researcher measuring outcome unaware of therapy received
• Often impractical or unsuccessful
Stratification
• Method for dealing with inequalities remaining even with randomization
• Subjects are grouped based on risk factors or co-morbid conditions capable of influencing the results prior to randomization
Matching
• Assignment procedure• Goal is to decrease/eliminate
selection bias• Study and control group are
selected to ensure that an identified variable is the same in both groups
• Ability to evaluate influence of “matched characteristic” on outcome is lost
Cross-over Design
• Uses subjects as their own control• Good for a small number of patients• Subjects assigned to all interventions and
measurements obtained for each treatment• Control for order effect (randomize)• Control for carryover effect
(sufficient wash-out period)• Blind the cross-over point
One Group Pre-treatment & Post-treatment Design
TreatmentMeasureOutcomeVariables
Population
XSampleMeasureBaselineVariables
Static Group Comparison
Sample
Treatment
Treatment
MeasureOutcomeVariables
MeasureOutcomeVariables
Population
X
Non-equivalent Pre and Post-treatment Control Group
Design
Sample
Treatment
Treatment
MeasureOutcomeVariables
MeasureOutcomeVariables
Population
X
MeasureBaselineVariables
MeasureBaselineVariables
Post-treatment Only Control Group Design
Sample
Treatment
Treatment
MeasureOutcomeVariables
MeasureOutcomeVariables
Population
R