(1) Overview of study design Flashcards
What are descriptive studies?
Descriptive studies involve answering three questions
1) How does the risk of developing the disease vary over time?
2) How does it vary from place to place?
3) How does it vary with respect to certain characteristics of the individuals, such as age, sex or occupational group?
What are analytical studies?
The determinants of the observed distribution may be investigated by asking one of three questions:
1) Do persons with the characteristic have the disease more frequently than those who do not have the characteristic? (Cross-sectional studies)
2) Do persons with the characteristic develop the disease more frequently than those who do not have the characteristic? (Longitudinal / Cohort studies)
3) Do persons with the disease have a characteristic more frequently than those without the disease? (Case-control studies)
What are experimental studies?
These involve asking the question: Does altering / removing the characteristic reduce the probability of developing the disease?
What types of epidemiological studies are there?
Observational - Where there is no input into the data it is just collected
Interventionl - Where there is control of the groups on what factors they’re exposed to
What is an ecological study?
- Analytical, describes outcome in populations or groups, and exposure
- Average exposure of a population vs. rate of outcome for that population
- Evidence of an association examined using correlation or regression
- Cannot comment on individual exposure and risk of outcome – “ecological fallacy”
Pros/cons of ecological studies?
Advantages:
- Cheap and quick
- Routine data available
- Exposure information available at area level
- Area differences in exposure may be larger than individual difference in one area
- Generates hypotheses
Disadvantages:
- Proxy measure of exposure as based on average for a population
- Area differences in recording or disease
- Systematic differences in exposure measure
- Data on confounders often unavailable
- Boundaries may inappropriately divide
What is prevalence?
The number of cases of a disorder present at a pomunt in time per recruited study population
What is a cross sectional study?
An analytic study in which cross sectional data (data at the time) is look at and monitered for people with/wothout the risk factor tested and seeing whether they have/don’t have the condition testing for
What is incidence?
The number of new cases
Cross secitional pros/cons
Advantages:
-Simple to conduct
-Quick
-Relatively cheap
-Frequently used for planning
-Estimate prevalence of common conditions of sufficient duration
-Estimates exposure in a population
Disadvantages:
-Not possible to answer whether outcome followed the exposure, or -exposure followed outcome
-No measure of incidence
-Not useful for rare disease (or exposure); too many people needed for adequate numbers
What is a cohort study (longitudinal)?
Measurements of disease and/or exposure are taken at 2 or more points in time.
Official definition:
An observational study where a population is studied for the presence of a characteristic (e.g. blood group) or exposure (e.g. smoking), which is thought to be a cause of a condition (e.g. cancer), prior to the onset of a condition. The entire population is followed up over time, and the incidence of disease in exposed individuals is compared with the incidence in those not exposed.
What is the point of cohort studies?
They ask the question: Do persons with a characteristic / exposure develop the disease more frequently than those who do not have the characteristic?
How do you work out relative risk?
Relative risk is the increased/decreased risk of developing a condition in the exposed grou and the control group.
Relative risk = Exopsed group/Control
How do you wrok out excess risk?
exposed group - control
Pros/cons of cohort studies?
Advantages:
-Direct measures of incidence & prevalence
-Multiple outcomes (& multiple exposures)
-Good for rare exposures
-Exposure measured prior to disease
Disadvantages:
-Bias if loss to follow-up
-Bias if outcome not assessed similarly in exposed & unexposed
-Inefficient for rare disease
-Expensive & slow
