ratio and risk Flashcards
relative risk
so many times more likely to get a condition if you are exposed
cohort study
compares disease rates
odds ratio
case control study
odds of exposure if you are a case
compares exposure not rate of disease
estimate of relative risk
How do you calculate an odds ratio
odds of exposure in cases/odds of exposure in controls
how do you interpret an odds ratio
ratio of 1 means exposure is no more likely in cases than controls
>1 exposure more likely in case group - implies might increase risk of disease
<1 exposure is less likely in case group - exposure might have protective effect
give an example of a null hypothesis
That the odds of taking HRT in women who had had an MI are the same as the odds of taking HRT in women who had not had an MI, i.e. the odds ratio equals 1. This would mean that taking HRT does not affect your chances of getting an MI (at least in the age range of those studied here)
how do you interpret a relative risk of 3.2 (95% confidence interval 3.11-3.29).
women 3.2 x more likely aged 50-70 than 30-50
Why would both relative risk and odds ratio be involved in the same study
odds ratio only a measure of relative risk
for rare outcomes measured over a long period of time or common outcomes the values will be different
What is the population excess fraction
a measure of the proportion of cases observed in the study population attributable to the exposure of interest
The population excess fraction (excess fraction of bladder cancer due to aromatic amine exposure in the whole study population) is 98.7 percent. How would you interpret this figure?
Assuming causality, 98.7 percent of the bladder cancer cases in the study population can be attributed to occupational exposure to aromatic amines.
Assume the same risks associated with occupational aromatic amine exposure (relative risk of ~297) and smoking (relative risk of ~5) in the occupational cohort apply to the whole population of England. In this England ‘cohort’, only 0.001% of the population has occupational exposure to aromatic amines, whilst 25% smoke. The population excess fraction is now 22.8% for aromatic amines, but remains at ~50% for smoking. Which exposure should be minimised to reduce incidence of bladder cancer in this population?
Although the relative risk associated with smoking is much lower than the relative risk associated with exposure to aromatic amines, the prevalence of exposure to aromatic amines is very low in this population. Assuming these exposures are causally related to bladder cancer, then of the ~10,000 new cases of bladder cancer in England each year, half of them (~5,000) could be attributed to smoking, and 22.8% of them (~2,280) could be attributed to occupational exposure to aromatic amines. It would therefore be more effective to reduce the number of smokers in the population than to minimise aromatic amine exposure (although obviously it would be best to reduce both exposures!).
What is the most useful measure of risk – the relative or the absolute (excess fraction) risk?
Depends on what you want to know
relative risk for risk factors
if you know it is causal - you can think about impact of the exposure on incidence of disease on population - attributable/excess risk
problems with attributable/excess risk
need to know it is casual
that there is no bias/confounding
if you want to extrapolate the data you need to know the sample is representative of the wider population
difficult to do all of above so it should just be a best guess of impact of exposure
percentages can = >100 if you calculate the fractions separately
what is the attributable risk for lung cancers in smokers
the rate of lung cancer amongst smokers - the rate of lung cancer amongst non-smokers
gives indication of how many extra cases for which the exposure is responsible
odds
another way to express a probability
odds = probability/|(1-probability)
rate and risk
person has a risk of getting a disease
population ahs a disease rate
both imply proportion
number of people with outcome of interest/total number of people at risk of outcome
