Healthcare Statistics
Chapter 2
Study Designs
Learning Objectives (1 of 2)
- List and define the components of a good study design
- Compare and contrast observational and experimental study designs
- Summarize the advantages and disadvantages of alternative study designs
Learning Objectives (1 of 2)
- Describe the key features of a randomized controlled trial
- Identify the study designs used in public health and medical studies
Study Designs
- Observational studies
- Case report/case series
- Cross-sectional (prevalence) survey
- Case-control study
- Cohort study
- Experimental studies
- Randomized controlled (clinical) trial
Inferences
- Observational studies—inferences limited to descriptions and associations; with carefully designed analysis, can make stronger inferences (statistical adjustment)
- Experimental studies—cause and effect
In all studies—need careful definition of disease (outcome) and exposure (risk factor)
Which Design Is Best?
- Depends on the study question
- What is current knowledge on topic?
- How common is disease (and risk factors)?
- How long would study take; what are costs?
- Ethical issues
Case Report/Case Series
- Observational study
- Case report—detailed report of specific features of case
- Case series—systematic review of common features of a small number of cases
- Advantage: cost-efficient
- Disadvantages: no comparison group, no specific research question
Case Series (1 of 2)
- Simplest design—description of interesting observations in a small number of individuals
- Usually case series do not involve control patients (i.e., patients free of disease)
- Usually lead to generation of hypotheses for more formal testing
- Criticisms: not planned, no research hypotheses
Case Series (2 of 2)
- Gottleib (1981) studied five young homosexual men with rare form of pneumonia and other unusual infections.
- Initial report was followed by more series (26 cases in NY and CA; “cluster” in southern CA; 34 cases among Haitians, etc.)
- Condition termed AIDS in 1982.
Cross-Sectional Survey (1 of 2)
- Observational study conducted at a point in time
- Advantages: cost-efficient, easy to implement, ethical
- Disadvantages: no temporal information, non-response bias
Cross-Sectional Survey (2 of 2)
- Is there an association between diabetes and cardiovascular disease (CVD)?
Patients with Diabetes
Patients without Diabetes
Patients with CVD
Prospective Cohort Study
- Observational study involving a group (cohort) of individuals who meet inclusion criteria followed prospectively in time for risk factor and outcome information
- Advantages: can assess temporal relationships
- Disadvantages: need large numbers for rare outcomes, confounding
Cohort Study (1 of 3)
- Is there an association between hypertension and cardiovascular disease?
CVD Hypertension
No CVD
Cohort
CVD
No Hypertension
No CVD
Study Start Time
Cohort Study (2 of 3)
- Identify a group of individuals that meet inclusion criteria.
- Follow prospectively in time.
- Assess exposure.
- Evaluate outcome status.
Cohort Study (3 of 3)
- Includes persons exposed and not exposed to risk factor at outset—usually persons are disease free.
- Can assess temporal relationship
- Problem if disease is rare (small numbers)
- Bias is less of an issue than in case-control.
- Confounding may be a problem.
The Framingham Heart Study
- 5000+ men and women enrolled in 1948
- Longitudinal cohort study
- Exams every 2 years for cardiovascular risk factors—surveillance
- Ancillary studies—hearing, exercise, nutrition, neurological studies
- 5000+ offspring and spouses enrolled in 1976
- Third generation enrolled in 2002
Selection of Study Sample
- Exposure group
- Common risk factors—general population (e.g., Framingham Study)
- Rare risk factors—special exposure cohort (e.g., soldiers exposed to agent orange)
- Comparison group
- Similar on all other factors that might affect outcome
Case-Control Study (1 of 3)
- Observational study involving individuals with (cases) and without (controls) outcome of interest
- Advantages: cost and time efficient for rare outcomes
- Disadvantages: need careful selection of cases and controls, bias
Case-Control Study (2 of 3)
- Is there an association between sleep position and sudden infant death syndrome (SIDS)?
Sleep prone
SIDS
Other
Sleep prone
No SIDS
Other
Study Start Time
Case-Control Study (3 of 3)
- Select subjects on the basis of outcome.
- Cases have disease.
- Controls are free of disease.
- Compare groups with respect to proportions with a history of exposure (possible cause).
- Investigation is retrospective in time.
Sampling
- Selection of cases
- Need explicit definition to make cases as homogeneous as possible
- Debate over whether cases should represent all persons with disease or specific subgroup (limit inferences)
- Selection of controls
- Should be comparable to cases (same exclusions)
- Controls represent non-diseased persons who would have been included as cases if they had disease.
Features
- Retrospective design
- Cost and time efficient
- Can get sufficient number of cases (useful for rare conditions)
- Can investigate array of exposures
- Best for diseases with long latency
Issues
- Ascertainment of exposure and disease status
- Both exposure and disease have occurred—hard to establish temporal relationship
- Bias
- Selection bias—select cases or controls and some drop out, leaving groups not comparable
- Observation bias—knowledge of disease might influence reporting of exposure (over-reporting among cases)
- Recall bias—retrospective (long term)
Randomized Control Trial (1 of 2)
- Experimental study where patients are randomized to receive one of several comparison treatments
- Advantages: gold standard from a statistical point of view, minimizes bias and confounding
- Disadvantages: expensive, requires extensive monitoring, inclusion criteria can limit generalizability
Randomized Control Trial (2 of 2)
- Is new drug effective in reducing hyperlipidemia (high total serum cholesterol)?
Hyperlipidemia Drug
No Hyperlipidemia
Sample RANDOMIZE
Hyperlipidemia
Placebo
No Hyperlipidemia
Study Start Time
Randomized Controlled Trial (Clinical Trial)
- Subjects are randomized to one of two (or more) treatments, one of which may be a control treatment.
- In the long run, treatment groups will be balanced in known and unknown prognostic factors.
- Important that the treatments are concurrent—that the active and control treatments occur in the same period of time
- Single- versus multicenter
Features
- If possible, a study should be double blinded—neither the investigator nor the participant are aware of what treatment the participant is undergoing.
- Sometimes it is impossible to blind the participants (for example, when the treatments being compared are medical versus surgical); but often it is possible to ensure that the people evaluating the outcome are unaware of the treatment.
Phase I: Safety
- First time in humans; main objective to assess toxicity and safety in humans—pharmacokinetics
- Usually involves 10 to 15 patients
- Subjects are usually healthy.
- Some are placebo-controlled.
Phase II: Feasibility Study
- Focus still on safety
- Side effects and adverse events
- Efficacy is important—goal is to determine optimal dosage.
- Involves a control group, and subjects are randomized.
Phase III: Clinical Trial
- Focus is efficacy.
- Data are collected to monitor safety.
- Involves a control group (placebo, active control)
- Usually involves 200 to 500 subjects
- Subjects are randomized.
- At least two centers
Phase IV: Post-Marketing
- After approval by FDA (based on efficacy proven statistically in two or more studies, New Drug Application (NDA) reviewed within 1 year)
- Focus is effectiveness.
Critical Components of RCT
- Randomization
- Control group—ethical issues
- Monitoring
- Interim analysis
- Data and safety monitoring board
- Data management
- Reporting