Lec 5-8. Measures of association Flashcards
What is the counterfactual theory?
All else being equal (in the same group) the outcome if the exposure didnt occur. In other words the best outcome of comparing groups would be comparing the group while they got exposed to the same exact group like they they never exposed. Its impossible.
What is exchangeability?
Comparability with respect to all other determinants of outcome. How the similar the two groups are.
Define absolute differences
Subtracting frequecies (counts)
Define Relative differences
Division (ratio) of frequencies (counts) or proportions
What is the difference between relative and absolute differences?
Absolute differences tend to be smaller and absolute differences deal with subtraction
Define risk
Proportion, simple part over whole. Outcome whether a person was exposed or not exposed.
What is the Incidence Risk (IR) equation for risk in exposed?
A/(A+B)
What is the Incidence Risk (IR) equation for risk in non-exposed?
C/(C+D)
What is NNT/NNH?
of pts needed to be treated to receive the state benefit/harm.
What is the equation for NNT/NNH?
1/Absolute risk reduction (ARR)
If the NNT is 3.03. How many pts need to be treated to recieve that benefit?
4
What value would we like our NNT to be?
Small
What value would we like our NNH to be?
large
What is the abbreviation for Risk Ratio?
RR
What is the equation for Risk Ratio?
(Risk outcome in exposed/Risk outcome in unexposed)
If the Ratio is >1.0 then the event/outcome is
more likely to occur. Increased risk.
If the Ratio is <1.0 then the event/outcome is
Less likely to occur. Decreased probability
If the Ratio is 1.0 then the event/outcome is
EQUALLY likely for both groups
If the Ratio is >2.0 then the event/outcome is
“x” times greater. Ex: OR=6.19, thus the comparator group is 6.19 times greater
When interpreting Ratio’s (RR/OR/HR) what three things do we look for?
Group comparison orientation, direction of words, and magnitude.
What is an example of group comparison orientation?
Ramipril vs. placebo OR placebo vs. Ramipril
What is an example of Direction of words?
increased/decreased
What is an example of Magnitude?
80% (1.8 times) or 20% (0.8 times)
When looking at the CI for Ratios (RR/OR/HR) if both values of the interval are on the same side of 1.0, what does that mean?
It is always Significant
What is the definition for odds?
Frequency of an outcome occurring vs NOT occurring.
What are the equations for Odds?
in cases (A/C), in controls (B/D)
What is the equation for Odds ratio?
[(AxD)/(BxC)]
