Case controlled studies Flashcards
What are the main criteria for assessing caussation
Bradford-Hill criteria:
Strength of association (relative risk, odds raito), consistency, specificity (single), temporality, dose responsse relationship, biological plausibility
coherence, exxperimental evidence, analogy
why are reports of association between genotypes and outcome so inconnsistent
variation of underlying asssociation between genotype and outcome between populations
heterogenous phenotypes
confounding by population stratification
failure to exclude chance as explaanaation
publication bias
What are the advantages+disadvantages of case control studies
Advantages: Inexpensive and quick
Good for rare outcomes and multiple risk factors
Can look at risk factors in detail
Disadvantage: not good for rare exposures, prone to selection, recall bias, uncertainty of exposure-disease time relaitonship, inability to provide direct estimate of risk, not efficient for studying rare exposures
what are the features of a case control study
retrospective, obtained from past/information that is lifelong
observational
no follow up=quick aanswer
suitable for rare diseases-all accessible cases can be located and controls found afterwards
What does the odds ratio do
estimates ratio of disease odds in exposedd group to disease odds in unexpossed group in study population
how to work out odds of event happening in exposed group and unexposed
exposed: pe/1-pe
unexposed=p0/1-p0
If there is no association existing between exposure and disease stauts
Odds ratio should bee equal a/b=c/d
How to calculate odds ratio
ad/bc
What does it mean if odds ratio is greater than 1 and less than 1
greater=increase in risk
less=protective or reduced risk
What does it mean if 95% CI contains 1 or Odds ratio=1
95% contain 1: little evidence against null hypothesis
Odds ratio=1: no difference beetween exposed and non exposed population
What does 95% CI mean
95% of the time confident that the interval includes true population parameter
What does P value=0.05 mean
1/20 likelihood that something is occuring by chance
What are the main types of bias
selection bias (case/control/exposure/sampling/response), information bias (misclassification of source and type), confounding, admission bias (higher chance of admisssion doesn’t equate to prevalence of disease), diagnostic bias (more likely to check up if knowledge of certain activities), survival bias (only survivers can enter study), non response bias (people who can’t be contacted may have higher/lower OR), recall bias, interviewer bias
What is confounding
situation wheere association between exposure and outcome is entirely/partially due to another exposure
what are the pros and cons for cohort study
pro: exposure measured before disease onset, allows multiple outcomes per exposure including incidence, rare exposure by selection, offer advantages less recall bias
cons: insufficient for rare diseases, prospective (expensive and time consuming\, retrospective (inadequate redcords), validity can be affected by loss to follow up
