L20: Measures of Association Flashcards
What are the measures of association?
- relative risk (risk ratio and rate ratio)
- odds ratio
How do we construct a contingency table to present the data?
Exposure yes no as row and outcome yes no as column
How to express RR and OR in words?
Those who were exposed have x time the risk/odds of developing the outcome compared to those unexposed.
When is risk ratio used?
Used in cohort study when all subjects are followed for the same duration of time
How to calculate cumulative incidence?
a/a+b ; c/c+d
How to calculate risk ratio?
cumulative incidence in exposed group / cumulative incidence in unexposed group
When is rate ratio used?
Used in cohort study when subjects have diff lengths of follow up.
How to calculate incidence rate?
a/total person time (exposed)
c/total person time (unexposed)
How to calculate rate ratio?
incidence rate in exposed group / incidence rate in unexposed group
When is odd ratio used?
Typically used in case control study.
In case control study, relative risk cannot be calculated directly. Subjects identified based on outcome status -> cannot calculate incidence
How to calculate odds ratio?
cross product = ad/bc
When is odds ratio a good estimate of risk ratio?
When the outcome is rare (<10%) –> use case control study
What do we do if there are multiple exposure groups?
Select the least exposed group as the reference group
What does the confidence interval of RR or OR tell us?
- statistical significance of the association
- precision of the point estimate (RR or OR)
How to tell statistical significance of RR or OR?
If CI does not include 1 –> statistical significant association
If CI includes 1 –> association not statistically significant
