What measures are used to determine the strength of the relationship between an exposure and an
outcome? Effect of an exposure in causing an outcome? - Measures of association --> relative risk (RR),
odds ratio (OR), incidence rate ratio (IRR)
Measures of effect -->
i) in an exposed group: risk difference (RD), attributable proportion exposed
ii) in a population: population attributable risk (PAR), population attributable fraction (PAF)
What is relative risk? How is it calculated and interpreted? - Relative risk is a ratio of 2 probabilities and
is calculated as follows:
RR = P(D+|E+)/P(D+/E-)
An RR> 1 means that the exposed group is more likely to show the outcome than the non-exposed
group while an RR<1 means the exposed group is less likely than the non-exposed group to show the
outcome.
What are odds ratios? How are they calculated and interpreted? - Odds ratios are ratios of 2 odds and
are calculated as follows:
OR = (odds of disease/exposure in the exposed/diseased group)/(odds of disease/exposure in the non-
exposed/non-diseased group)
or : OR = (ad)/(bc)
In cohort studies --> an OR >1 means that the odds of disease are greater in the exposed group than the
non-exposed group
, In case-control studies --> an OR >1 means that the odds of exposure are greater in the diseased group
than the non-diseased group
Under what circumstances do the RR and OR closely approximate each other? - The RR and OR closely
approximate each other when the outcome of interest is a rare disease.
What are incidence rate ratios? How are they calculated and interpreted? - The incidence rate ratio is a
ratio of incidence rates between 2 groups and is calculated as follows:
IRR = incidence rate of disease in exposed group/incidence rate of disease in a non-exposed group
or: IRR = (a1/t1)/(ao/to)
An IRR >1 means that incidence rate of disease in the exposed group is greater than the incidence rate in
the non-exposed group
What is risk/rate difference? How is it calculated and interpreted? - Risk/rate differences measure the
difference in risk or rate of the outcome in the exposed and non-exposed groups.
Risk difference = P(D+|E+) - P(D+|E-)
Rate difference = (rate in E+) - (rate in E-)
The RD>0 represents the excess risk/rate of the outcome in the exposed group.
What is the attributable proportion exposed? How is it calculated and interpreted? - The AP measures
the proportion of risk or rate of the outcome in the exposed group that is attributable to the exposure.
For risk data: AP = risk difference/P(D+/E+)
For rate data: AP = rate difference/(rate in E+)
The AP represents the % of the outcome in the exposed group that is caused by exposure.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller VasilyKichigin. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $13.48. You're not tied to anything after your purchase.
