Case Control Studies Flashcards
What do case control studies involve?
- identify individuals with a disease (cases)
- identify similar individuals without the disease (controls)
- determine previous exposure
- relate information on exposure to disease
How do you select cases for a case control study?
- representative of all people with the selected disease of interest
- could be incident cases from disease registry
- could be hospital based recruitment but may be biased
How do you select controls?
- same population as cases
- if hospital based ensure reason for being in hospital not related to exposure
- usually more than 1 control per case
Why and how should cases and controls be matched?
- know about potential confounders (age/gender)
- if using GP register can select by age and sex
What is undermatching?
- cases and controls not similar enough
- different smoking ages
What is overmatching?
- cases and control too similar
- sibling testing
What are the types of bias in case control studies?
- recall bias: cases may remember more than controls
- reverse causality: disease caused changes in recent exposures
- selection of cases: representative of all people with disease
- selection of controls: representative of all people with disease
What are nested case control studies?
prepared case control studies
- collected blood samples from larger population, frozen, then wait to see who gets disease
- cohort with disease split into exposed and unexposed
- cohort without split into exposed and unexposed
What are the advantages of nested case control studies?
- cheap, quick, easy
- exposure before disease
- can retrieve stored samples to look at new biomarkers
What are the disadvantages of nested case control studies?
- need cohort study with stored serum samples
Why can you not use relative risks when analysing case control studies?
- do not know the risk of the disease as you start with cases with the disease
What are odds ratios?
- used to analyse case control studies
- odds exposure in cases/odds exposure in control
- good estimate of relative risk if disease is rare
- if ratio is 2 = babies who died twice as likely to have been put on their sides rather than their backs compared to babies who did not die
What is odds exposure in cases?
Number of exposed cases x number of unexposed controls
What is odds exposure in controls?
number of exposed controls x number of unexposed cases
What is absolute excess risk?
Risk in exposed - risk in unexposed
What is attributable proportion?
incidence in population attributable to exposure/incidence in population
What is a cross sectional study?
- measure existing disease and current exposure
- sample at 1 point in time without knowledge of disease or exposure
What are the advantages of a cross sectional study?
- can look at exposures that won’t change (gender)
- gives measures of prevalence and exposure rates
What are the disadvantages of cross sectional studies?
- no use for rare exposures or rare disease
- not useful for assessing casualty
What is the risk of bias in different types of studies?
BIAS LIKLEY - cross sectional - case control - cohort - clinical trials BIAS NOT LIKELY
What is the strength of proof in different types of studies?
WEAK cross sectional case control cohort clinical trials STRONG
What is the Bradford Hill criteria for causation?
- strength of association
- dose response
- time sequence
- reversibility
- biological plausibility
- coherence of evidence in other study types
- consistency of findings in other populations
Why may exposure and disease be associated?
- bias
- chance
- casual
- incorrect analysis
- confounding
- reverse causality
