How do I explain risk to patients? Flashcards
Coursera Week 3
What is risk?
In stats this refers to the probability that an event will occur in a group, whether it be a good or bad event
What is probability?
It is an estimate of the chance of an event occurring divided by the total number of chances available
What are odds?
The ratios of a player’s chance of losing to their chances of winning
OR
The average frequency of loss to the average frequency of win
Why do we use odds?
It is helpful when we want to compare how much larger one probability is relative to another
i.e. imagine we bought three tickets the probability of winning would be 75%, the odds would be 75 to 25 = 3 to 1 so winning is three times more likely to occur
How are risk ratio and odds ratio used in a clinical setting?
RR is more useful for explaining how successful a treatment is at preventing events like fracture or stoke, whilst OR is useful for a capturing the rare side effect
When is a risk ratio used?
To compare the risk in two different groups of people, i.e. smokers and non-smokers
Usually used in prospective cohort studies
When is an odds ratio used?
As a measure of association between an exposure and an outcome, i.e. smoking and cancer
Usually used in case-control studies and sometimes in cohort studies (when incidence is less than 10%)
When is an absolute risk used?
For an individual risk of developing disease over a time time period, i.e. smokers and cancer
Usually used for the number of events that occurred in a group, divided by the number of people in that group
What is relative risk?
The ratio of the probability of an outcome in an exposed group compared to the probability of an outcome in an unexposed group
Can be expressed as a percentage decrease or a percentage increase
How is relative risk calculated?
A/(A+B) / C/(C+D)
A = outcome 1, group 1 B = outcome 2, group 1 C = outcome 1, group 2 D = outcome 2, group 2
What do the following values of relative risk mean:
a) RR = 1,
b) RR < 1,
C) RR > 1?
a) exposure is not associated with the outcome
b) risk of the outcome is decreased by the exposure
c) risk of the outcome is increased by the exposure
What is an odds ratio?
The odds are the ratio of the probability that an outcome occurs to the probability that the outcome does not occur
How is an odds ratio calculated?
(AD) / (BC)
A = outcome 1, group 1 B = outcome 2, group 1 C = outcome 1, group 2 D = outcome 2, group 2
*cross product calculation
What do the following values of an odds ratio mean:
a) OR = 1,
b) OR < 1,
C) OR > 1?
a) exposure is not associated with the outcome
b) lower odds of the outcome happening with the exposure
c) higher odds of the outcome happening with the exposure
What are 95% confidence intervals used for?
To estimate the precision of the RR and OR
It is the range of values that can be 95% certain contains the true mean of the population
