STUDY DESIGN - LEARNING OUTCOMES Flashcards
Describe the main features of case reports and series.
Most basic type of descriptive study and documents an individual’s medical experience. A clinician may see an interesting case and describe what he/she has seen. A case series is an extension of a case report and is based on a small group of individuals. These studies are useful for hypothesis generating, but because there is no comparison group, their use is limited and statistical relationships between exposure and outcome cannot be assessed.
Describe the main features of ecological studies.
Ecological studies look at the association between exposure and disease on a population or area level rather than on an individual level. With ecological studies any associations seen are on a population level, and we cannot assume that this transfers to an individual level. To assume that associations seen on a population level apply to the individual level is called the ecological fallacy.
Describe the main features of cross-sectional studies.
A cross-sectional study collects observations on people at one point in time, thereby providing a ‘snap shot’ of the health of the population. Cross-sectional studies only provide information about disease prevalence and not disease incidence. Study subjects should be selected so they are representative of the target population.
Describe the main features of case-control studies.
The case-control study design is useful for looking at risk factors for rare diseases. However, it cannot look at prevalence or incidence of disease. It only looks at whether associations with exposure exist. In a case-control study cases are selected on the basis or presence or absence of disease. A group of individuals with the disease are selected (cases), along with a group of individuals without the disease (controls). Disease frequency cannot be measured because subjects are chosen or sampled according to their disease status.
Describe the main features of cohort studies.
Cohort studies measure the incidence of disease. In the traditional prospective cohort study, the study population is ‘free of disease’ at the beginning and the exposure variables are measured and then the population is measured through time to determine their disease outcome, and to compare the risk of disease in those who are exposed and those who are unexposed.
Describe the main features of randomised controlled trials.
Controlled trials are human experiment where an investigator implements the intervention (or exposure) or interest and determines which individuals are exposed and unexposed. Randomisation is often used within trials to ensure that the exposed and unexposed groups are similar with respect to all other factors (including other risk factors for the outcome). An RTC is thus equivalent to a laboratory experiment where the investigator changes one variable at a time, keeping all other variables constant.
When is it appropriate to use case reports and series?
Case reports and series are useful for generating a hypothesis.
When is it appropriate to use ecological studies?
Ecological studies are a good first step in investigating possible exposure-disease relationships especially when there are restraints on time and money.
They are good for hypothesis generating.
They are also good when investigating an exposure which has little variation between individuals within a population/area, but large variations between populations/areas (e.g. some dietary factors).
When is it appropriate to use cross-sectional studies?
Cross-sectional studies are good for measuring prevalence of disease. Only measure prevalent (i.e. current) cases of disease.
Good for measuring some continuous measure such as blood pressure or level of a protein in serum.
Gives a snap shot of the health of a particular population at a particular point in time.
When is it appropriate to use case-control studies?
Case-control studies are useful for looking at risk factors for rare disease.
A case control study is appropriate when you have a single disease of interest because that is how we are doing our recruitment - it is more difficult to look at multiple outcomes than it would be for a cross-sectional study for example.
When the disease may be rare and you would have to use a very large cross-sectional or cohort study to get anybody with your outcome of interest.
It is useful when we are only looking at associations rather than trying to assess prevalence.
Also useful when looking at exposure(s) that are relatively common.
When is it appropriate to use cohort studies?
Cohort studies are appropriate when you want to measure the incidence of a disease. This would traditionally involve starting with a cohort that is free of disease and following them through.
When is it appropriate to use randomised controlled trials?
Randomised controlled trials are appropriate in cases where you want to test the effect of a certain exposure in comparison to a control exposure or placebo.
List the advantages and disadvantages of case reports and series.
Advantages:
- Allows reporting on unusual medical cases
- Results used to generate hypotheses
- Generates evidence of possible new disease
- Rapid feedback of current events in medical community
Disadvantages:
- Cannot be used to assess statistical associations
- Could just be reporting a medical oddity
List the advantages and disadvantages of ecological studies.
Advantages:
- Inexpensive and quick to conduct
- Exposure and disease information is often more readily available by area
- Differences in exposures often greater between areas than between individuals in one area
Disadvantages:
- Results cannot be extrapolated to the individual level
- Systematic differences between areas in recording disease frequency can occur
- Quality of diagnosis
- Differences in classification
- Completeness of reporting
- Sampling of the population can distort the results of an individual
- Data often not available on confounders
List the advantages and disadvantages of cross-sectional studies.
Advantages:
- Can be used to examine how much disease there is in a population, and to look at cross-sectional associations between exposures and disease
- Can look at more than one disease and more than one exposure
- Can be relatively inexpensive and quick to conduct and drop-out is not a problem as no follow up
Disadvantages:
- Disease and exposure are measured at the same time therefore no temporal association can be made
- Not suitable for studying rare exposures or rare outcomes
- High possibility of recall / reporting bias
List the advantages and disadvantages of case-control studies.
Advantages:
- There is no need to follow people up over time which can be costly, time-consuming and subject to drop-outs
- It reduces the number of disease free people that need to be studied – this is particularly important when the disease of interest is rare
- It is possible to study multiple exposures
Disadvantages:
- No estimate of disease frequency can be obtained
- Bias can be a problem, particularly selection bias
- It is possible to look at one outcome only
List the advantages and disadvantages of cohort studies.
Advantages:
- Measures incidence of disease
- Multiple disease outcomes can be examined relating to one exposure
- Often multiple exposures for one disease can be examined
- Exposure is measured before the development of the disease so we can assess the temporal relationship between exposure and disease
- Good assessment of level / duration of exposure and therefore allows us to look at dose relationship between exposure and disease
- Extremely useful for measuring the effect of rare exposures
- Selection / recall bias could potentially be very low
Disadvantages:
- Prospective studies are time consuming and extremely expensive
- Retrospective studies rely on the availability of good and accurate records
- Results can be effected due to loss of follow up (response rate)
- Can be inefficient for the evaluation of rare diseases
- Can be inefficient for exposure-disease relationships with a long latency period (i.e. it takes a long time for the exposure to influence the disease or the disease to develop after exposure)
- Ascertainment of outcomes could be influenced by exposure
List the advantages and disadvantages of randomised controlled trials.
Advantages:
- Rigorous evaluation of a single exposure or treatment
- Prospective design (can assess causation)
- Potentially eradicates bias and confounding
Disadvantages:
- Expensive and time consuming. This means that many trials are never done, or completed trials are too small or have follow-up periods that are too short. It also means that most trials are funded by large research bodies or drug companies (who may dictate the research agenda)
- There is a risk of hidden bias (e.g. did blinding really work?)
- Limited generalisiability – often exclude high risk groups such as the elderly, children or pregnant women
What are the design and analysis issues associated with case reports and series?
It is just a descriptive study and documents an individual’s medical experience. There is no comparison group so their use is limited and statistical outcomes between exposure and outcome cannot be assessed. Not really any analysis associated with case reports and series. Use limited - good place to start.
What are the design and analysis issues associated with ecological studies?
Confounding variables are a problem for ecological studies. To do something about confounding at the analysis stage you have to have collected the data and if you are using previously published data this may be an issue as that info may not be in there. The fact they we are dealing with a population level rather than an individual level can also cause problems.
We display ecological data as a scatter plot. Each dot on the scatter plot will represent a population or area. For example one scatter plot for an ecological data may have cigarettes per capita plotted on x, CHD deaths plotted on the Y, and then the dot representing the pop or area.
We need to think about confounding factors for apparent associations between two variables found in ecological studies.
We need to conduct analysis and interpretation of ecological studies at a population level and not an individual level. We have no data at the individual level. Without the information at the individual level often the correct interpretation of any relationship found can’t be established. If we want to extrapolate findings to the individual we would need to conduct further studies.
How do we display ecological data?
We display ecological data as a scatter plot.
Why do we use case-control studies for rare outcomes?
Because if we used any of the other study designs we would have to use large populations to pick up enough cases to analyse to look for associations.
What is the usual analysis method for categorical variables?
chi-squared test and generation of odds-ratios
Can you use the risk ratio in case control studies?
No
What is the purpose of a multivariate analysis?
A multivariate analysis takes into account more than one variable.
What could we deduce if our unadjusted and adjusted odds-ratios showed a big change?
We could deduce that the factors that were being taken into account for calculation of the adjusted odds ratio are having a confounding effect.
