Observational Studies Flashcards
What are the two type of study designs?
- Observational
2. Interventional
What are observational studies?
We just observe and participants do not change.
They are either:
- Descriptive
- Analytical
Give an example of a descriptive study
Cross sectional studies (surveys)
What is the aim of descriptive studies?
- Describe prevalence of disease (which is the number of cases in a population at a given time)
- How disease varies over time, place by place or characteristics of individual
Advantage of descriptive studies
- Able to identify patterns and trends to allow for further research/hypothesis - suggest clues to cause of disease
- Allow us to see burden of disease (impact of disease - financial, mortality and community)
Disadvantage of descriptive studies
- Does not provide sufficient evidence to infer causality as they are mostly retrospective and do not account for many details such as confounders
How can cross sectional studies be descriptive or analytical?
They can be descriptive - aiming to assess the burden of disease
They can be analytical - aim to explain observed pattern of disease by examining its relationship with possible aetiological factors
Most cross sectional studies have a mixture of both
Describe a cross sectional study
- Use to measure prevalence of a disease (via medical records, questionnaires etc)
- Carried out by collecting data (characteristic and disease status) at one point to see if subjects with certain exposure/characteristic have higher disease prevalence
- Can be descriptive and analytical
Advantages of cross sectional studies
- Suited to common diseases
- Relative quick to carry out and is cheap
- Can give estimates of prevalence of disease
Disadvantages of cross sectional studies
- Unable to measure incidence (just prevalence)
- Not suitable in rare diseases as a very large sample is needed
- Need to ensure that you avoid selection bias
- Exposure/characteristics may change in time or disease progression
- Reverse causality (information on exposure preceding disease is unknown). This occurs as exposure/characteristics and disease status are measured at one point in time so its hard to tell what came first.
Examples of analytical studies
- Longitudinal/cohort studies
- Case-control studies
- Cross-sectional studies
What is the aim of analytical studies?
The aim is to examine associations between the presence of diseases in individuals and populations with potential causative factors.
Advantages of analytical studies
- Gives equal information on both controls (free of disease) and cases (those with disease)
- Gives greater clues ti causality than descriptive studies
Disadvantages of analytical studies
Still unable to infer causality due to possible confounders/unknown variables
Describe a Longitudinal/Cohort Studies
- Analytical study
- Used to see if those with certain characteristic who do not have disease go on to DEVELOP the disease more frequently than those who do not have the disease
- Subjects are divided into two groups (those who have characteristic and those who do not)
- When measuring exposure status of subjects they must be free of the disease under investigation
- Two types: prospective cohort study or retrospective cohort study
What are the two types of Longitudinal/Cohort Study?
- Prospective Cohort Study
2. Retrospective Cohort Study
Advantages of Longitudinal/Cohort Studies
- No recall bias as it doesn’t rely on subjects memory
- Exposure is measured before causality, therefore less bias and increased reliability in causality
- More than one disease can be measured for any one exposure
- Can calculate incidence rates
- Can calculate relative risk
- Potential to give more information leading to nested case control study
Disadvantages of Longitudinal/Cohort Study
- Can take a long time and be expensive
- Collection of data may alter behaviour - observer effect
- Ensure definition of exposure or disease is consistent
- Losses by follow up may introduce selection bias (patients who are healthy leave)
- Not suitable for low incidence disease
What is a prospective cohort study?
Population is studied for the presence of a fixed or modifiable exposure, the population is followed up and the incidence of disease in exposed individuals is compared with the incidence of those who are not exposed.
What is retrospective cohort study?
Past medical records are used to identify a population and obtain data on previous exposures. The incidence of disease in exposed individuals is compared with the incidence in those not exposure.
Compare retrospective and prospective cohort studies
Retrospective cohort studies are cheaper and less time consuming
Describe case control studies
- Subjects are divided into two groups -those who have disease and those who do not
- This aims to identify cases (with disease) and controls (without disease) and then measure the prevalence of a particular exposure
- This aims to see if cases had a characteristic more frequent than the control
- If exposure is more common in cases than controls, it may be a risk factor
- If exposure is more common in control than cases, it may be a protective factor
What does risk factor and protective factor mean?
If exposure is more common in cases than controls in case control study it is a risk factor.
If exposure is more common in control than cases in a case control study it is a protective factor.
Advantages of case control studies
- Cheap and quick to do
- Efficient for rare diseases or disease with low incidence
- Can investigate a range of risk factors
- Can work out the odds ratio - which is even more accurate in rare diseases and can be interchanged with relative risk
Disadvantage of case control studies
- Prone to selection and information bias, such as recall bias as the exposure is measured after the disease develops
- Can’t calculate relative risk as there are no cases of incidence
- Not suitable for exposures/ characteristics that are rare
- Not suitable for diseases with several main exposures/ risk factors
- Need to ensure data collection is not influenced by knowledge of exposure to prevent measurement bias
- Disease may affect exposure having a reverse causal relationship.
What is a nested case-control study?
This is when a case-control study is inserted into a cohort study.
A subset of controls (from cohort study without disease) are compared to incidence cases (from cohort study - those who developed the disease).
The cases of cohort study become the nested case control study.
