Flashcards in HaDPoP general Deck (29)
What is a consensus useful for?
- Allocation of resources
- Projections of populations
- Trends in populations Eg Ethnicity or age
How are confidence intervals calculated?
Lower bound = Value ÷ Error factor
Upper bound = Value x error factor
(error factor equation given in exam)
Cohort studies can either be?
1) Prospective - disease free individual recruited and followed up
2) Retrospective - Disease free individuals recruited then exposure status calculated from historical documentation and followed up
Explain internal comparisons
Occur when you have sub-cohorts within your original group and then compare exposed and unexposed within the cohort
Explain external comparisons
Occur when you have your exposed population compared against a reference population instead
Use an SMR calculation (removes confounders)
Describe healthy worker effect
Healthy worker effect is whereby there is biasing of results when a study involves workers / employed individual compared ti a reference population and is a form of selection bias
When a comparison is made, it should always be against other working individuals to prevent any bias
Describe a case control study
A case-control study involves recruiting disease-free (controls) individuals and diseased individuals (cases) and then their exposure status is determined
What biases affect case-control studies?
- Selection bias
- Recall bias
What are the positives and negatives of a cohort study?
+ Good to study rare EXPOSURE; adequate numbers of people can be picked from the population, where a small number is exposed
+ Opportunity to look for different potential OUTCOMES at once from varying exposures
- Expensive and time consuming, especially if the disease has a long latent time period Eg AIDS
+ Allows for calculation of specific absolute risk
What are the positives and negatives of a case-control study?
+ Good for rare diseases; no need to follow thousands of individuals to get a few cases
+ Opportunity to look for different potential EXPOSURES at once, as long as detailed background can be obtained
+ Cheap and quick
- Can not obtain absolute risk (unless nested case-control)
- Heavily affected by recall and selection bias
Why do randomised controlled trials use randomisation?
Remove any confounders that may be present in the study, known or unknown
What is the Bradford-Hill Criteria used for?
To determine whether a causal-effect relationship has been established (once confounders, bias and chance have been removed), Bradford-Hill criteria can be used to evaluate the relationship
The more Bradford-Hill criteria present, the more likely it is to be a causal-effect relationship
What are the 9 Bradford-hill criteria
1) Strength of association
2) Specificity of association
3) Consistency of association
4) Temporal sequence
5) Dose response
7) Biological Plausibility
8) Coherence of theory
Bradford-Hill criteria. Strength of association
Stronger associations (Eg high IRR or OR) are more likely to be causal
Bradford-Hill criteria. Specificity of association
Outcome is associated with a specific factor
Eg mesothelioma caused by asbestos exposure
Bradford-Hill criteria. Consistency of association
Association occurs in other studies too
Bradford-Hill criteria. Temporal sequence
Causative factor precedes the outcome
Bradford-Hill criteria. Dose response
Different levels of exposure lead to different levels of outcome
Eg radiation levels
Bradford-Hill criteria. Reversibility
Removal of causative factor causes reduced risk of outcome
Bradford-Hill criteria. Biological plausibility
Biological mechanism to support the theory
Bradford-Hill criteria. Coherence of theory
Observed observation confirms current scientific thinking
Bradford-Hill criteria. Analogy
Another similar disease has similar outcomes
Describe the ethics of a randomised control trial
Clinical Equipoise – Reasonable uncertainty into which drug is better for the patient, so not subjecting patients to known less effective treatment
Scientifically Robust – Persuit of knowledge for the good of the general population
Ethical Recruitment – Recruitment for region where drug will take affect and no unethical exclusions from the trial
Valid Consent – Participants given sufficient knowledge, cooling off period, chance to ask questions, and ability to withdraw from trial at any point
Voluntariness – No coercion or manipulation into entering the trial
+ The ethics of any medical roles
What is a systemic review?
A compilation of primary studies
What is a meta-analysis
Within a systemic review , a meta-analysis provide the quantitative synthesis of the primary studies used in the trial
provides an overall value with associated confidence intervals
What is a forest plot?
A graphical representation of a meta-analysis
The horizontal line corresponds to the 95% confidence interval
The Vertical line corresponds to "line of no effect"; intervention has no effect on the outcome
Overall effect is given to give best estimate of all the data analysed together
What two types of models can be used in a meta-analysis
1) Fixed effects model - assumes the studies used are homogenous and any variation between data comes from within-study variation
2) Random effects model - Assumes the studies are heterogenous and variation between data comes from within-study variation and between-study variation
In a funnel plot, what does a typical funnel shape indicate?
A well balanced systemic review